The rules are so easy, a drunk person could understand.
Take one of the cans and shake it. Now I don't mean just shake it, I mean SHAKE the sucker! Till it's about to blow up. Then put it in the box with the other cans and mix them around (one person not looking) and then switch them around again (the other person not looking) Basically, nobody knows where it is.
Now, one person picks a can, holds it to his/her head at an angle, and opens it. If it's not the one, s/he has to drink it. If it is the one, s/he gets a wet head and you can start all over or whatever. Then, if it wasn't the one, the next player takes one and tries. etc.