Summarize:

Set up your game by placing an empty glass or plastic cup (the titular "King's Cup") in the center of the table. Spread an entire deck of cards, jokers removed, in a circle around the cup, face down. You'll have a full ring of cards around your chalice when you're ready to play.  Everyone playing should be in a circle around the cup and should be able to easily pick a card on their turn. If you are drinking out of cans, you can place an unopened can in the center instead of a cup. This variation is also a bit more sanitary since only one person will need to drink from the can instead of a communal cup. Each player needs their own beverage in front of them, and they will drink from it or contribute to the King's Cup as the game goes on. Each card in King's Cup, when drawn, means that someone has to take a sip of their drink, so if someone's drink runs out they should refill it or get a new one. You do not have to all have the same type of drink, though it does make the King's Cup, a mixture of everyone's drinks, much easier to swallow. Each action forces someone to drink. Usually, this is a casual, single sip. However, some people like to make rules for each drink, something along the lines of "3 seconds per drink," or some other amount of time. King's Cup has a very specific set of rules which all of the players need to be familiar with before playing. Each turn, one person will draw a card from the deck, and each card is associated with a simple action for someone at the table. You should feel free to adapt or edit the rules for your game, since there are countless variations and rules, but a common set includes:   2 is for you. If a player draws a 2 (of any color or suit) they may choose another player who then has to drink. The chosen person must drink for the duration of time agreed on at the start of the game.  3 is for me. If a player draws a 3, they must take a drink themselves.  4 is for floor. If a player draws a 4, everybody at the table has to reach down and touch the floor as quickly as possible. Last one to touch the floor drinks.  5 is for guys. If a player draws a 5, all the guys at the table must drink.  6 is for chicks. If a player draws a 6, all the girls at the table must drink.  7 is for heaven. If a player draws a 7, everyone at the table has to put both hands in the air as quickly as possible. Last one to do so must drink.  8 is for mate. If a player draws an 8, they have to pick another person at the table who must drink every time they do, and vice versa.  This continues until someone else draws an 8. If either of the players forgets to drink when their "mate" is drinking, they must take an additional penalty drink.  9 is for rhyme. If a player draws a 9, they must pick a word and say it out loud, such as "rat." Moving clockwise, each player must say a word that rhymes with the original word, e.g. cat, hat, bat and so on, and they must do so in under 5 seconds. This keeps going round the table until a player can no longer think of a rhyming word. That player drinks.  10 is for "Never Have I Ever." If a player draws a 10, everyone at the table must hold up three fingers. Starting with the person who drew the card, that player must begin a sentence with "never have I ever..." and complete the sentence with something they have never done. If other players at the table have done that particular thing, they must lower a finger. This continues around the table. The first player or players with no remaining fingers must drink.  Jack is "Make a Rule." If a player draws a Jack, they have the ability to make a rule that must be followed for the entire duration of the game. They could rule that nobody is allowed to swear, that it's forbidden to use the bathroom, or that nobody can call anyone by their first name. Any player who breaks the rule must take a penalty drink. Read popular rules ideas below.   Queen is for Question Master. If a player draws a Queen, they become the Question Master. Until another player draws a Queen, everyone can only respond to the Question Master's questions with another question. For example, if Terry is Question Master, and asks Steve "what time is it?" Steve would have to respond "Is it 2 o'clock?" If Steve doesn't answer with a question, he must take a drink.  King is for King's Cup. When a player draws a King, they must pour whatever they are drinking into the cup in the center of the table. When the fourth King is drawn, that player must drink the concoction in the cup, signaling the end of the game. If you're playing with a can, Kings could mean "finish your current drink," or you could make an alternate rule.  Aces are for waterfalls. If a player draws an Ace, everyone at the table must chug their drink, starting with the player who drew the card. Each player may only start drinking when the person to their right has started drinking and may only stop when the person to their right has stopped drinking. So if you're sitting to the left of the player who drew the card, you cannot stop drinking until everyone at the table has stopped. Choose a player to begin, then draw a card at random. Turn it over quickly and then act out the card according to your rules. Once you're done, discard the card and move on to the next player, who draws. If playing with a can, slide the card underneath the tab of the unopened drink. As cards build up, eventually the can will crack open. The player who "cracks the King's Cup," then needs to drink the entire can. There are hundreds of variations for each card, but there are also a few other rules that you can make that will spice up the game. Most of the rules are made to get people drinking more frequently:   Ring of Fire: The game is played as normal. Fan the cards out so all of them overlap. Any player who breaks the circle has to finish their current drink.  Color Kings: For all the numbered cards (not Jacks, Queens, or Kings), someone drinks for the number of seconds on the card. "Red is to the Head," meaning you drink for the number of seconds. "Black gives Back," meaning you pick who has to drink.
Place a cup in the center of a table and surround it with a deck of cards. Pour everyone an individual drink. Determine the "punishment" length. Decide on the rules for each card. Pick up the first card. Up the stakes with new rules.