You'll need a deck of cards.
To begin with the players must decide who will start out as the dealer, and whether aces are high or low.
The dealer holds the deck of cards face down and asks the player on their left to guess the top card. If the player guesses correctly the dealer must drink, otherwise the dealer tells them whether the top card is higher or lower than the card they guessed at. The player then guesses once more. If this second guess is correct then the dealer must drink, otherwise the player drinks a number of fingers equal to the difference between their guess and the actual card.
The dealer then repeats this whole process with the next player, and so on. If the dealer gets through three players in a row without having to drink then they pass the deck to the player on their left, who is now the dealer.
Cards which have been revealed are placed face up on the table so that players know which cards are no longer in the deck, making it harder for the dealer as the game goes on.