Fill a pot with water 2 to 3 inches deep and place it on a stove burner set to high until it reaches a rolling boil. Fill a bowl with lukewarm water and place it off to the side.
Place the mouth guard in the boiling water and allow it to heat up for about one minute, or for the length of time recommended on the package.
Use a pair of cooking tongs to remove the mouth guard from the boiling water and dip it into the bowl of lukewarm water for about five seconds.
Place the mouth guard over your teeth immediately after removing it from the bowl and clench your teeth around the plastic. Hold your jaw tightly around the mouth guard for about one minute so the heated plastic can form to your teeth.
Remove the mouth guard and place it in a clean storage container or on a clean paper towel so that it can harden fully. This process can take one to two hours ,depending on the specific mouth guard.