first, you make (or have your mommy make) a ball of play dough... smaller usually works best when you're just learning the game.
next, you take your hand and smash it onto the ball. the nice thing about a small ball is that you don't even have to press very hard... which is important if you're a bit hesitant about touching said play dough.
finally, remove your hand and see the amazing results. it's FLAT! and sometimes, if you're really lucky (or have recently eaten something with jelly), the play dough might even stick to your hand!