Starting with the player to the left of the dealer and going clockwise, each player plays a card in front of any player. The collection of cards in front of a given player is their “stack.”
There are three rules about playing cards:
The first card a player plays must be on another player. That is, a player can’t play their first card of the round on themself.
The first card on a particular target goes face-up. The second and subsequent cards on a given target are played face-down.
Once a player has four cards in front of them, no more cards can be played on them. Players can only play cards on targets who don’t have four cards yet.
To be clear, after the first time around the table, players are free to play cards on themselves. (As long as they don’t already have four cards in their stack, of course.)
Players keep playing cards until all of the cards dealt this round have been played. The dealer should be the last to play.