Fill the pot half full of water. Place the pot on the stove.
Turn the stove on medium to medium high, depending on your particular stove's settings. Allow the water to begin boiling.
Carefully drop the blank mouth guard in the water. Allow it to sit for 30 seconds.
Use the tongs to pull the still warm mouth guard from the water. Now touch it--if it is too hot, allow a moment for it to cool down a little.
Place the mouth guard in your mouth and clench your teeth. Hold this for 30 seconds to allow the warm rubber to mold itself around your teeth.
Take the mouth guard out and run cold water over it from the sink. Once has solidified again, place it back in your mouth to see if it fits properly.
Dump the water from the pot and turn off the stove if the now custom fitted mouth guard fits well. If not, repeat Steps 1 through 6 again to allow for a re-fit.