Boil a pot of water.
Fill a bowl with ice water.
Grasp the mouth guard with a pair of tongs. Submerge the mouth gard in the boiling water for 30 seconds.
Pull the mouth guard out of the boiling water and immediately plunge it into the ice water for one second.
Grab the mouth guard with your hand and place it in your mouth, around your upper teeth. Bite down hard. Press up against the mouth guard with your tongue to help mold it against the roof of your mouth. As you press, create suction in your mouth by sucking all of the air and water out of it. Keep pressing for 30 seconds.
Dip the mouth guard back into the ice water for 30 seconds to set the plastic.