Before each hand, the player to the left of the dealer puts in the big blind, the player to their left the small. Any action always begins with the player to the dealer's left, moving clockwise.
Using the deck shown below, each player is dealt four cards, followed by a round of betting.
One shared card is turned; another round of betting.
This is repeated twice, so that each player holds their original four cards, and three shared cards are on the table.
One remaining card is turned, called the Snap, and each player is given an opportunity to exchange a card from their hand for the Snap (or whatever card is in its place if someone does exchange).
The Snap is then discarded, a final round of betting ensues, and the player with the best hand wins.
History of the Game at Hogwarts
While Exploding Snap is fitting for school clubs and all ages, Sevens involves gambling, which is technically against school rules. The game is generally permitted when the stakes are without value, i.e. playing for pebbles or twigs, etc., but not for money, quills, sweets, etc.
Often, Professors have been nostalgic for their own student years and turned a blind eye to the occasional mention of a discreet game, so long as things don't get out of hand. In the past, games have been busted up (or students given detention) when there are reports of drinking, smoking, drugs, other serious rule breaking, or when they become common knowledge to the student body (and are therefore publicly flouting the rule). When things go well, it's actually one of the best displays of setting aside house prejudices, as anyone is welcome to play so long as they don't start trouble or tattle. Leave your baggage at the door, this is just about the cards.