You'll need a deck of cards.
Choose a player to be the dealer for the first round. After each round the role of dealer passes to the next player. A round consists of the dealer placing cards face up in a pile, one after another, calling out the number of each card as they do so.
When two cards of the same value come up one after another the first person to place their hand on the pile and shout 'Snap!' can nominate another player to drink.
A round ends when the dealer has no more cards.