One of the cups of each pair is empty and is placed upside down. The other is filled with beer and is placed next to the empty cup. If your opponent hits your empty glass you drink once from your full glass. If he hits your full glass, he has to drink his whole full glass. It seems simple, but I guarantee you'll get drunk quick.
Accuracy diminishes as the game wears on.
You can be super accurate most of the time, but the one time your not could be deadly, especially if your opponent is a terrible shot. That will make sense when you start playing.