The first step to crushing a can on your head is to take the end that you drink out of and place it in the center of your hand. Make sure that your hand covers the end with the tab on it
Step 2: Placing your fingers on the can
This part is imprtant, because if you skip this step, its going to hurt and you probably won't crush the can
Put your fingers around the can, with the tips of your fingers pointing into the side of the can
Step 3: Count to three
To start, you want to raise the can to your head, and then pull it back, counting once each time you raise the can. As you count, slightly add more force on the sides of the can with your fingers. You should do this three times, counting out loud.
Step 4: Crushing the can
After you've counted out loud to three, hit your head with the can, simultaneously pushing in the sides of the can with your fingers. The combined force of your fingers pushing on the side of the can and your hand pushing the ends of the can into your head will crush the can. This is how the whole sequence should look.