The plastic will feel very warm, but it will not burn you. When your mouthpiece has cooled, take it out and dip it into a glass of cold water for about 10 seconds to set the custom shape. Then place the mouthpiece back into your mouth to evaluate the fit. You want to feel your lower jaw being held in a comfortable forward position. If you are not happy, repeat steps 4-8.
Most people get a more comfortable fit by molding it a second time.
You may remold at least 3 times. If you determine a remold is necessary, only reheat for 8 seconds. Remove the holding tab by cutting it off with scissors as close to the mouthpiece as you can. If you don't stop snoring completely, re-mold with your jaw extended a little farther, but not at maximum extension. The first couple of nights you should only use it for part of the night.