Fill a saucepan with 3 to 4 inches of water. Place the pot over high heat and bring the water to a boil.
Pour 2 to 3 inches of cold water into a bowl. Set the bowl on the counter near the stove for quick access as the mouth guard will need to be transported quickly from the hot to cold water.
Turn the stove off once the water in the saucepan boils. Allow the water to stand for 30 seconds.
Place the mouth guard in the hot water for 60 to 90 seconds. Avoid leaving the guard in the heated water for more than 90 seconds or the integrity of the shape and structure may become ruined.
Remove the mouth guard from the hot water with a slotted spoon or metal tongs. Place the guard in the cold water bowl for one to two seconds to cool the material.
Moisten your lips with saliva or lip balm to reduce friction before inserting the mouth guard. Put the guard in your mouth and around your upper teeth. Look in a mirror to make sure that the guard is properly aligned; the front center should be lined up with your front teeth. Bite your molars firmly into the mouth guard first, followed by the front teeth.
Bring your lower jaw forward to align the bottom teeth with the top teeth. Bite down hard into the mouth guard so that it forms to the shape of your mouth. Press your fingers against your cheeks to help push the mouth guard against your teeth. Hold for 20 seconds.
Release your bite and remove the mouth guard from your mouth. Run the guard under cold water for 30 seconds, which helps it to hold the current shape.
Place the mouth guard back in your mouth to test the shape. Repeat the steps if the guard does not fit properly in your mouth.