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Best way to learn is to do, in my experience. I've DMed for a number of groups that have never played before (with players from age 11 to age 56(!!)), and in my experience, they really struggle to understand the game until they play it.

Step one is to get everyone a character sheet-- if they're really lost, make one for them, or get a pre-made one from online. There's a lot of numbers that go into a character sheet that are confusing when you have no idea what any of them are used for. Give your players as much support as they want, here; even if they're perfectly capable of creating a character sheet, they may feel more comfortable with an optimized character made by someone else. Or, if having control over the race and class of the character for roleplay purposes is important to them, do a little handholding. However much they end up needing, go for it.

Step two is just to play. That's it. Tell them that the basic concept of the game is that they say what they want to do, and that you, as the DM, will tell them how it happens, and that they might need to roll some dice to determine that.

I like to start with simple things. In my one-shot with new players, I just had them go directly into a simple fight, and let them go at it, since that was all they were going to do during the one-shot. In my longer campaign, I created a small "tutorial level", letting my players wander a city for a little bit and make some purchases and do some minor roleplaying, then had them go into a small battle, where all of them had the chance to make a couple of attack rolls and deal with being attacked in turn.

People will (generally) get the hang of the game as you play. (I say generally, because I've been playing with a friend who plays clerics a lot, and she still doesn't know how casting spells works.) I think the best way is to get rid of as much of the explanation as you can, and just toss them all in.

The other thing that really helps (but that you can't really make them do) is listening to or watching other people play. Even rules-loose podcasts still give a general sense of how playing the game works. I found it to be very helpful to my abilities and confidence as a new player, and there's a clear difference between my players who have listened to or watched DND campaigns, and the ones who haven't.

I DM for new players alot, and even play with my wife and 8 year old daughter. Boredom is the enemy, even with seasoned players. If a player gets bored, you lose them. Keep things moving, even if it means handwaving some things.

I make a point of trying to know as much about the rules as I can, so my players don't have to. I mean, I want them to pick up the rules, and know what to do next; but I don't want them to HAVE to know. I want them to be able to say "I want to do X", and be able to say, "great, to that roll a D20 and add..."

Session 0 for New Players

I don't session Zero for parties of new players. I talk to them individually, and ask them to tell me about the character they want to play. I introduce the idea of class and race, and suggest one that matches the vauge ideas they may already have.

Like if I hear "I want to have a magic sword and be really tall"; I'll tell them "that sounds like a Goliath Paladin. A Goliath is a small giant race, and Paladins are holy warriors or principled knights that infuse their weapons with 'smites' to make their attacks more powerful."

I won't have the roll stats, or pick spells. I'll give them one of hte WotC pregrens, or I'll do character creation between meeting with them, and the first session. I'll use the standard array and put stats where it makes sense based on what they told me about their ideas. If it is a wizard, warlock, or sorcerer I might have them pick spells with me during the meeting; but other subclasses that choose spells I'll typically grab what makes sense with their narrative and stats wise.

If they end up liking the game, but not the character; they can build a new one after the first session.

Don't Lecture

What I try to do, is take a "learn as you go" approach. similar to how they learn video games. I start by saying something along the lines of:

This is like a video game and choose your own adventure story put together. Except you can TRY to do anything. I'll describe what the world around you is like, and you tell me what your player tries to do in response.

I make sure to make clear that the sheet of paper is your character sheet, it's the numbers about how good they are at different stuff. It is just information they will need at times, but don't worry about that until I ask you to do something with it. Then

Jump Right In (Video Game Style)

I jump into the prologue for the first session.

So, you are sitting in a run-down tavern, the smell of old spilled ale perfumes the air. It's a crowded tonight, and the house band is starting a new set. You have the feeling you are being watched.

Like a video game tutorial, I slowly introduce concepts, by giving them an opportunity to try it.

Do any of you look around to see if you can find out why you have this feeling? Great, you two do, you're going to make a skill check for perception. This is a 20 sided die, when you make a skill check (or an attack) you'll roll this and then add your "modifier" for the skill from this box here to see how well you do. What did you roll? And then, add that. So, you got a 3, you see there are a lot of people in the tavern. And you, you got a 15, you can see a hooded figure in corner is watching your table with interest. They look away when you look at them. What do you do?

Combat

Even if you did lecture, the first combat with new people takes a long time, and they are level one so the first enemies they run across are likely going to be a monster for every party member, and all be 1 HP AC 10 or AC 12 creatures. I want the monsters in this first fight to die in one hit, but there to be a alright chance that someone might miss the first time. This is one point I pull out my regular games favorite minion minis -- gummy bears. Whoever kills it, eats it.

You stumble on some really small angry green goblin looking creatures that come running out the forest at you. They appear to be ready to attack you. We're going into rounds now, so I need you all to roll initiative. We'll do this for any combat situation and some special challenges, too. To do this, you roll your 20 sided die, again we call it a D20 for short, and add the number in the initiative box, right here on your sheets. Everyone has to roll, and I'll roll for the monsters.

That first combat, even though it is only single hit characters, will take a long time. You should explain up front that this combat will be slow because everyone is learning, but during other people's turn is a great time for them to think about what they might do on their turn. Letting them know on the first person's turn about the action economy. Use the game terms for things as they happen, let them get used to hearing "action, bonus action, movement." Remembering to ask, "Do have a bonus action?" and "Do you want to move anywhere?" and letting them know "If you move, you'll be open to the monster taking an attack of opportunity."

At level one, the options are easy for most of the players as all a lot of characters can do at level one is "I hit it with my X" or "I cast firebolt/Eldritch Blast at it". Spellcasters may have a lot more to consider, but they will still have one "go to" attack. Even still, remember to give color to the attack, like "You slice down into the green rubbery skin...", or ask for color "What does your firebolt look like?" to keep the game feeling like the game even with the added tutorial.

That ended up longer than I meant.

1. Step One: Explain As You Go

There are a few things that are need-to-know from the outset-- character stats, hit points, the other basics of combat (armor, rolling to hit) and, for spell casters, their spells.

But other than those, the detailed mechanics can probably wait until such time as they come up naturally in the game. Players will be primed to receive and retain information when it is directly pertinent to what they're doing in the game.

Honestly, even stats can wait for a while if you take things sufficiently in hand that you make their characters for them. This might sound like a stretch, but it's perfectly possible to get a sense of the type of character someone wants to play just by asking them in something between a conversation and an interview, and then making a good faith effort as GM to make a character under those guidelines. (I know, I've done it. Not for this purpose, but it worked out fine.)

2. Step Two: A Structured Sequence of First Encounters

You can also take an idea from modern CRPGs, which typically lack anything in the way of printed instructions at all. Partly this is because the computer knows the rules for them, and partly because many games have an introduction mode where increasingly complex game features are introduced, highlighted, explained, and commented on.

So it is quite possible for you to start them out by making characters to their details and explaining them a little bit, and then having (say) a social encounter where you introduce them to social mechanics rules and interacting peacefully with NPCs; then a wilderness trek sequence where some of those rules are introduced; then a combat sequence where you explain the basics of combat; etc.

This does take some forethought on your part, because you're also implicitly (or explicitly!) telling the players what you think is most important and where your focus will be. This is probably something you want to design, to a certain extent, rather than just winging it. You do have to decide what order things go in so they make sense, and also they reflect the game you want to play.

K.I.S.S. - Keep it Simple Silly

I tell beginners or people interested something along these lines:

• It's interactive story time. One person tells a story and everyone else is one of the protagonists.


• It's like a video game, except one person creates the world and everyone else is a player. They can interact and explore as they please and generally go on an overarching adventure.


• It's like improv acting in a fantasy adventure setting.


• Remember playing "imagination" when you were a kid? It's that with a lot more structure.

A few more things I always add:

• We use dice to determine the results of actions in the game.


• You can make of it what you want and take it as seriously or playfully as makes it fun for you.


• There are a lot of rules. A lot. But don't worry, because I know them all or know where to find them. And I'll be the story teller. You just do what you want and I'll figure out how to fit it into the rules. You'll pick up the important ones quickly and you can learn as many or as little as you like.

Keep everything simple as you move forward.

Lastly, it doesn't hurt to just run a couple battles with no story before you and they commit to an adventure. The players will pick up a lot of how the game mechanics work from having their ranger fight a couple monsters. Keep it light and fun, and make sure they win!

Start with basics and play rather than explain

Don't explain, just play.

The play's the thing.

A good approach is to do the first few sessions in the Theater of the Mind. No maps. No figures. Just character sheets, a few dice, you, and your players. Have them introduce themselves in character, tell them about whatever adventure the party will be doing using a brief description and a hook (save the cattle herd, find a kidnapped child, search for a gem in an old crumbling tower in the forest, etc)

Set up situations and employ the three basics of the game (Basic Rules, 5e, page 3)

1. The DM describes the environment.




2. The players describe what they want to do.



3. 
   

   The DM narrates the results of the adventurers’ actions. (Roll dice only when necessary to resolve an action or where the result is in doubt).

   
   
   

   Rinse and repeat.

   
   

A few key points:

1. The important thing: always have them tell you what they are going
   to do, or trying to do, and then have them a die roll if needed.




2. For spell caster players, a little coaching by you, the DM, on how
   a spell works is valuable when starting out.




3. Answer questions as they come up, rather than trying to explain everything up front.

Objective: have fun.

When in doubt, the dice be damned and apply the rule of cool. If it's fun or funny, roll with it.
Keep the pace of play moving.

This approach is similar to ExTSR's shock treatment for a "rules light" approach to D&D as explained here. I've used an approach very like that with beginners: it works. Per that example, keep the pace of play moving. When you need to make a ruling and keep the pace going, err in favor of the players.

This is a time tested way to play. As told by Rob Kuntz and others who first began playing with Gary Gygax in the game's formative years, it was common to have very few rules, and a group of players clustered around the table in Gary's office where only his voice was heard. He'd describe the scene and the players would tell what they were doing. (He'd be hiding behind his file cabinets and describing either what they saw or what happened). They'd sometimes roll some dice to see what happened, other times, the narration didn't need a die roll.

Last year I was a complete beginner - I still am - I gave up because the sessions were weekly and nothing, absolutely nothing happened in the first two sessions. We spent the whole time doing paperwork that we didn't understand the reason for. We wanted to get out into a fantasy land and have adventures. It was more like taking on a new office job where you were required to fill in forms without knowing what they were for or how they'd affect us when we eventually got to play.

If only we'd been allocated a set of preset throwaway characters to experiment with and allowed to dive right in. The DM could have let us try stuff, explore caves, fight monsters, told us to throw a die and then interpreted whether we succeeded or failed instead of explaining in detail why we had succeeded or failed.

What I'm saying is that it was obviously boring for the DM to have to go through all the fine details with people who (a) didn't understand it and (b) didn't want to understand it. It was boring for us as well because we just wanted to get playing in order to discover whether we even wanted to get involved long term.

By the third week we we had had a battle with some orcs or something and the DM made them seem about as alive as shop dummies. It seemed like one stroke of a sword followed by ten minutes maths.

Anyway, that's my rant over. Just be aware that some people are there simply wanting to live in a fantasy land and have adventures and don't want to start by going back to a schoolroom.
Once people have experienced the game and start to get a feel for the rules, then they will be willing to start again and create their own characters properly.

To make my personal recommendations clear:

1. Have a selection of pre-generated characters. Preferably several more than there will be players. Make sure the abilities are reasonably distributed so no-one is immensely strong or uselessly weak.




2. Allow players to choose the type of character they are emotionally drawn to without being concerned with abilities. If there is a conflict for the same character, resolve it with a simple competitive throw of dice until a winner emerges.




3. Give them a very simple and obvious task to complete.

For example: You are trapped in a cell with stone walls and a solid oak door. Your job is to escape. After they have experimented with the environment, either they will find a secret passage or they will kick up a racket to attract the guards and try to overpower them. The chances are this will take a whole session but escaping or nearly escaping will give them a desire to continue exploring a gradually expanding world.

4. Get them used to throwing a die to see whether they succeed. If they try one at a time to climb up to a window, let them. If they all fail then don't let them try again. Just say it's proved impossible and they'll start exploring other strategies. They won't question your authority to say, e.g. "You all try to reach the window and realise that if you keep trying you will simply become more tired."




5. When this initial baptism has occurred they can be given the option to keep their preset characters or create new ones.

Summary

Have an expanding world where both the terrain and the rules are gradually discovered organically at the same time. NPCs need to have a function in life. For example everyone understands guards to a cell. These can be standard RPG creatures but they have a function rather than just attacking for no reason only to get slaughtered by a group of novices.

Consider the D&D Starter Set.

Having DMed for many new players (in various editions of D&D and in numerous other systems), I observe that their main problem is always the same: being overwhelmed. Everything is new. Even before engaging the rules, just learning the vocabulary of a game can be a huge challenge. Moreover, the scope of the game and what players can do within it usually starts off unclear. Many new players suffer from choice paralysis from too much open-endedness too soon. They get stuck thinking "what can I do?" instead of "what should I do?"

Perhaps seeing those same problems, Wizards of the Coast published an introductory product for 5e called the D&D Starter Set. It includes pregenerated characters, a simplified rulebook, and a positively excellent low-level adventure called Lost Mine of Phandelver. In my experience, LMoP does everything right for an introductory game:

• It is purposely written to connect to the hooks built into the pregen
  characters, so the players have a sense that their characters belong
  in and to the world.


• It puts the players in media res, immediately giving them a
  direction and a purpose. Players have no time to feel paralyzed by an
  open-ended world; they're in danger right now!
  


• It presents a series of scenarios structured to ease the players into
  the rules bit by bit. The very first encounter introduces fundamental
  concepts like simple skill checks, initiative, turns, actions, range,
  and stealth. The first dungeon adds traps, lighting, cover, difficult
  terrain, and so on. This way, the rules are gradually cemented in players' minds in a learn-by-doing model, instead of remaining abstract conceits with no practical referents.

Finally, players can play through all of LMoP with just the freely-available Basic Rules -- which means you can distribute those rules to all of your players and let them read and familiarize themselves at their leisure.

Make patience your watchword.

Of course, no product, however well-made, can take the place of a DM's single most important teaching tool: patient repetition.

Being a DM means repeating rules over and over. I have veteran players who still have to ask me, again and again, why the party rogue doesn't get advantage on account of having an ally within 5 feet of her target enemy, or why the wizard's familiar can't end its turn in his space. Heck, I myself frequently have to go back and reread rules. At the end of the day, D&D is a ludicrously complex ruleset. (The volume and depth of 5e questions here on RPG.SE is certainly testament to that.) Everyone has to learn it at his or her own pace. If you stick with it and give them what they need, even if it means repeating yourself a thousand times, eventually your players will catch on.

How do I explain RPGs (D&D or others) to beginners?

"We're going to explore an imaginary world. You each play a person in
that world and tell me what you want to do, then I tell you what
happens as a result."

Add in a quick overview of the setting. Ask them what kind of person they want to be in that setting, then either select pregen characters (with a little tweaking) or create new ones myself (if we're having this conversation prior to game night) based on what they want.

Depending on my feel for the people involved, I might give them an actual character sheet or I might just give them a general description of the character and its strengths/weaknesses; I'll keep a copy of the actual character sheet myself either way, so that I can handle all of the character's mechanics myself without needing to make the player deal with them.

The entire point of this is that there is absolutely no need for new players to know any of the rules. Yes, this seems like it would increase the burden on the GM by making them manage all the rules for everyone, but the first time I tried it, I quickly realized that I was doing that anyhow - I always double-checked players' calculations, vetted their application of the rules for correctness, etc., so it didn't really change much for me and it freed the players from having to master the rules if they didn't want to.

Depending on the players' interest and your own feelings about managing everything yourself, you can gradually introduce them to the rules over the course of the first several sessions, but that's still not necessary. I've GM'd for groups of experienced players who knew the rules, but still preferred to play with no player-facing mechanics, not even rolling dice themselves. But, on the other hand, if your group or system are of the sort to emphasize things like charop or other forms of rules-mastery-focused play, then you'll need to transition at some point from this mode of play into one where players are more directly involved with the rules.

I frequently run games for new players. I usually don't spend much time explaining the game. Instead, I say something like this:

Here's your character sheet (one of the pregen character sheets from the official website). This game is based on rolling a d20 -- this die right here.

If you try to do something, I might ask you to make a skill check for how well it works. To do that, you roll your d20 and then add your skill bonus. Skill bonuses are on this section of your character sheet. Higher numbers are better.

If you can't find a skill bonus on your list, you can use a stat bonus instead. Stat bonuses are on this section of your character sheet.

If you get into combat, you might need to attack something. Here's the part of your character sheet that talks about how you attack things. You'll roll that same d20 and add your attack bonus, which is this number here. If you hit, you'll roll damage, which is this number here.

That's all the time I spend on explaining rules. Usually the player then spends some time reading through their spells and abilities; sometimes they'll ask me to explain how a spell works, and I'll do that.

Here's my thought about this: the most important part of D&D is not understanding the skill system and the magic system. The most important part is listening to the DM telling you what's going on, and answering with a creative response. If you spend too much time on the rules, it will just get in the way of playing the game.

It is incredibly easy to overcomplicate D&D for new players. Unless you're using pregenerated characters, the setup time takes a while and there aren't a lot of ways around that.

The reality is that you have a series of basic stats with simple modifiers that you use when rolling your dice, you have some standard item profiles, and that's it. The problem is that a character sheet, especially one you have to fill in yourself, is a really clunky way to start and it saps the enthusiasm like a black hole.

The most recent game I've been taking part in took two sessions and some online instant-messaging to sort out our character sheets. A lot of it was just looking up things in the rules, like weapon and equipment profiles.

As DM, your job is fundamentally to facilitate the game, so do everything in your power to alleviate the paperwork.

Right off the bat, if I were to DM a game (and I might, we've been going around the group DMing short games in our own various styles) I'd be using cards.
My weapons, spells and equipment are totally standard, so why am I writing them down when I could have a deck of common cards with their individual rules?

This simplifies more than half the character sheet away and compartmentalises the information, reducing information overload. It also makes it easier to add and remove items from a character as the game goes on.

You can extend this concept, representing character buffs and debuffs as tokens. Our current game includes the concept of Fatigue as a negative modifier to some or all of our rolls. Having to rub out and rewrite the same stat multiple times per session is going to get old fast — far easier to just take a "fatigue token" and set it on your character sheet in front of you. Different coloured glass gem-stones can be bought in bulk; those would do the job fine.

There is a free PDF of equipment/status/spell cards. It's a pretty hefty PDF, but it might be a good starting point!

It's also worth colour-coding the stats for readability if you can. Before you print the character sheets off, use a free editor like paint.net or similar to tint all the skill listings to match the core stats they're derived from. Nothing too complex.
It sounds kinda stupid but doing this will help your players' eyes track better and make the wall of numbers a lot easier to read.
Frankly, I might grab some coloured pencils and do it to my current character sheet before our next session.

Little Quality-of-Life stuff like this will alleviate a lot of the small problems that turn it from a game into a chore, which is at the root of the problems you're running into.

I take from your question that you guys are not just new to D&D but to pen&paper roleplaying games in general. In such case, that is a lot to take in.

From my 25 years of experience, here are two things I wish I had done from the beginning:

Skip the rules

Focus on the roleplaying part. Ignore the rules. Make it clear this is an introductory session and the real game will be a little different. Let everyone pick characters simply by announcing ("I want to be a mage!"). Then DM entirely by feeling. No dice rolls. Just play and adjucate. DM word is final. No rules, no dicerolls. If the mage wants to throw a fireball, let him and announce what happens, without damage tables, spell slots, all that nonsense.

You want to get into the basic structure and feeling of a roleplaying game. You don't want to be bogged down by rules and tables. Keeping the intro adventure simple helps a lot. Village needs saving from a few goblins stealing food at night or something. There is enough time for dragons and crazy magic later.

Let them read the rulebook

If anyone shows the slightest interest in the rules themselves, let them read the rules, borrow them the book. The more people have actually read the rules, the better.

For example, in one of my earliest groups we had one player with an unbelievable memory for numbers and tables. He would know by heart every table in the rulebook. "how much impact damage for a fall from 60m?" "...uh... +3" saved us hours of time looking things up. :-)

Progress to D&D

Ok, after some people are familiar with the rules, and everyone has played a roleplaying game one or two times, then you can introduce the actual game system to be played.

I'm sure there are plenty of intro adventures to D&D available, with pre-made characters. Pick one of them, it will save you tons of time and they are often made to introduce people to most of the rules while playing.

Focus on the core rules, nothing exotic or additional. If you don't remember a rule while playing, go back to the golden rule and make something up. DM word is still final. I've always made a house rule that says any rules discussions are to be postponed until after the game. While playing, a player can make one comment, such as "but I'm an Elf, don't I get +2 on this roll?" and if the DM rules otherwise, no further discussion, until after the game. Be firm on that during the game, and very open and flexible after. After the game is when the DM should freely admit to every mistake he made.

Keep it flowing.

The Setting

Stick to standard fantasy stuff everyone is familiar with during the first few sessions. Then slowly introduce D&D specific things (monsters, items, places, etc.)

We had a trope during our first games that no matter who was the DM (we took turns), the game would inevitable always start in the middle of a forest, and there would be some path to the side... - you can always have a forest, a cave, a ruined castle or some isolated village without knowing much about the setting.

With those steps, you introduce people slowly to the basic concept of playing, then to the rules, then to the specific setting, without overwhelming them at any step. You also from the start get them used to the DM being the final arbiter, which smoothes gameplay. Nothing breaks the mood like an extended rules discussion (and you wouldn't believe how many things simply aren't important enough to be actually brought up after the game).