Fill a pot with water and place it on a stove-top. Turn on the stove's heat until the water reaches its boiling point.
Lower your mouthguard into the boiling water. Allow it to remain there for at least 30 seconds. Remove the mouthguard from the pot with a pair of tongs and place it into a cold cup of water for 3-to-5 seconds.
Take the mouthguard out of the water and put it in your mouth. Bite down for several minutes. Place mouthguard in the cup of cool water until its shape has solidified.