Mao Card Game Rules
Setup
I would say that the game is best played with 4+ people, and you need one card deck for every 2-3 people, *maybe* every 4 people if it's a really large group. They just all get shuffled together, and as long as you have enough decks, you can play with as many people as you want. The amount of cards to deal can either be 5 or 7, depending on how you feel. 5-card games go quicker, but 7-card games means more penalties and opportunities to learn rules. The goal is to get rid of all your cards.
In Mao, literally the only thing you're allowed to tell new players is, "I'm not allowed to tell you the rules, but you won't be left completely in the dark because I will penalize you with a card and the rule you broke when you do break any. Also, you can't talk about the rules at any point today or in the future." You and anyone else playing who knows the rules have to make sure to pay attention to cards being played so that you can enforce the rules, and then like I said, give them a card and state what rule they broke. If the rule pertains to an incorrectly played card, you give them that card back as well as a penalty card. Some rules you'll see have to be followed before gameplay can actually continue, so you keep penalizing them every few seconds until they do it. Technically, you're also supposed to keep penalizing people until they follow non-gameplay-important rules, but I don't like to do that until getting through a few games first. Keep in mind that some rules can overlap, so they'd have to follow all of them before their turn can end. Any missed rules are 1 penalty card each, as you might expect. Finally, players do also get penalized for following rules when they *don't* need to, and people handing out penalties also need to be penalized for incorrect penalties (to make this easy, you can just give them the card that they handed out). Technically, anyone can hand out penalties, but this rule is why you should feel confident about the rules before you do.
Start
Players are not allowed to touch their hands before the game begins, which is always a funny way to set the tone with new players because they'll probably pick up their hands after you deal. In order to start the game, point at the person you want to lay the first card, spin your hand in a circle in the direction you want play to move, and then flip over the top card of the deck to start the discard pile. While you're doing that, say "the game of Mao begins now." After the game begins, you can touch cards. The main rule to be followed during gameplay is that no talking is allowed, which is also funny to hit new people with. In addition, one small but important rule I'll mention now is that cards are not allowed below the table for any reason, whether it's an accidental drop or just holding your hand down there.
Gameplay
As for gameplay and the cards themselves, it's kind of like Uno. You must follow suit, or you can play a different suit of the same number. However, if you play a same number/different suit card, you must knock as an indication of the new suit. In addition, if the next person also changes the suit with the same number card, they must knock twice. Knocks continue to scale by 1 for each continuous suit change. After someone breaks the streak and doesn't change the suit, knocking would start back at 1 for the next time it needs to happen. Playing an ace skips the next person, playing an eight reverses the direction of play, and jacks are wild. Some people allow any jack to be played on any suit, but the way I've always done it is that the jack must follow suit. No matter what you do for playing the jack though, you then must call a new suit (or the same suit if you want). When you play a seven, you have to say "Have a nice day", and when you play a spade, you have to name the full card (i.e. "ace of spades"). If you don't have a card to play, draw a card, and if you can't play that card either, just say "pass". If someone takes too long to play a card (use your best judgement), whether because they don't know they're supposed to or they forget or whatever, they get penalized for "delay of game".
End
Whenever a player has exactly 1 card in their hand, they must say "last card". When they play their final card, they must still carry out any rules they need to, and only after all rules are carried out can they say "Mao" and end the game. If they miss any rules, they would still get penalized and gameplay would continue. The winner of the game gets to do 2 things: start the next game following the rules above (this is the only time you're actually allowed to tell someone a rule if they don't know how to start the game), and they also get to add a new rule to that specific session of Mao. New rules would continue to stack during a single session after each win, but you would go back to base rules at the next session. As long as the new rule doesn't conflict with the base rules and it isn't something completely insane (like draw 4 or pushups), then it's allowed. Keep in mind that whoever made the rule would be the only one to know it at that point, so they would be the only ones able to enforce it at the beginning.
Point of Order
Something anyone can do at any point during the game if they really need to talk is call "point of order". In this state, touching cards is once again not allowed, but talking is allowed. Only the person who started point of order can end it by saying "end point of order". As soon as that is said, gameplay continues, which means you can touch cards again, and no more talking.
Edge Cases
There are a couple of things I've run into before that you should know about. First, if a jack is played, and the next person plays a jack of the suit that was called, then they don't need to knock because even though the jack on the table is a different suit, they are following the suit that was called. However, if you play a jack on a jack that's not the suit that was called, then you still need to knock. Second, since "last card" must be called anytime 1 card is in your hand, that means if someone gets penalized for not following a rule with their last card, then giving them that penalty card would mean there is now 1 card in their hand. Therefore, they must call "last card". Otherwise, you would also need to penalize them for not saying "last card".