Immerse the mouth guard in a deep mug filled with boiling water. Leave it in until the material softens and is flexible.
Place the mouth guard in your mouth and bite down as hard as you can to mold it the shape of your mouth. Hold the bite for at least 20 seconds. Use your tongue and fingers to help mold the piece to fit as comfortably as possible.
Look for teeth marks and impressions in the mouth guard interior. Repeat the heating process if the impression is faint. You must have a proper impression for the mouth guard to be worn correctly.
Once it is shaped correctly, immerse the mouth guard in cold water to harden it.
Place the boil and bite mouth guard in your mouth.
Set your top teeth into the top of the mouth guard.
Adjust the guard with your teeth, tongue and fingers.