The players both count to five (three), though it is obviously better to repeat the name of the game (Monkey! Robot! Pirate! Ninja! Zombie!). Each time you raise your fist and swing it down. On the fifth (third) count, you form your hand into one of the five gestures. (It is recommended that in addition to the hand gesture, you also add an aural component to this).
See Also: Rock-Paper-Scissors-Spock-Lizard
via Boing Boing