How to Fit a Mouth Guard
To fit a mouth guard, start by putting it in your mouth and trimming any excess material off of the ends of the guard. Then, submerge the guard in boiling water for 30-60 seconds to soften the plastic. Remove the guard carefully from the water with a slotted spoon, and then put it in your mouth. Use your thumbs to push the guard against your back molars, and bite down firmly for 15-20 seconds to get a good fit. Remove the guard from your mouth and place it in a bowl of cold water for 1-2 minutes to solidify the plastic.
Gather the necessary materials. To properly fit your mouthguard, you'll need: your mouthguard scissors enough boiling water to submerge the mouthguard in a bowl of ice water a towel Trim the mouthguard for length. You may need to trim the ends to make sure your mouthguard fits comfortably and doesn't irritate the back of your mouth. Put it in before you fit it and trim a bit off the ends if necessary. If it pokes into your mouth at the back of your jaw, or causes you to gag, trim off a little using scissors. A mouthguard is mostly used to protect your vulnerable front teeth, not your molars, so you've got some wiggle-room as far as the back of the mouthguard goes. Some athletes prefer a more comfortable and short guard that covers the teeth as far as the first molars. Do what's comfortable. Submerge the mouthguard in boiling water. You need enough boiling water to submerge the mouthguard in it for between 30 and 60 seconds. Start a small pot of boiling water on the stove, or boil some in the microwave. Holding the mouthguard carefully by the end strap, submerge it in the water and let it soften. If your mouthguard doesn't have a strap, or you already cut it off, you can just drop it in the water and retrieve it with a slotted spoon. If you have braces, or other dental prosthetics, boil the water for only around 30 seconds. You want the mouthguard to fit your mouth, but not to fill in the crannies around your braces, which can damage them. Remove the mouthguard carefully. Quickly blot the mouthguard on the towel and get it into your mouth, adjusting it against your upper teeth. It won't be uncomfortably hot. Using your thumbs, push the mouth guard up and back against your molars. Bite down firmly to get the impression of your teeth on the bottom of the mouthguard and suck the guard against your top teeth. Place your tongue against the roof of the mouth to create pressure and achieve a tight fit around your teeth. This shouldn't take more than 15 or 20 seconds. Don't chew the mouthguard or move it around in your mouth while you're fitting it. Place the mouthguard in the ice water. Remove the mouthguard and dunk it in the ice water. Let it cool for a minute or two and try the fit. It should fit snugly against your top teeth without having to hold it with your tongue, and it should fit naturally into your bottom teeth. If you want to cut off the strap, you can do so now, or remove the removable strap. If it feels uncomfortable or ill-fitting, try it again until you get a proper fit. 
Gather the necessary materials. Trim the mouthguard for length. Submerge the mouthguard in boiling water. Remove the mouthguard carefully. Place the mouthguard in the ice water. 