Do they fit as well as professional trays made from the dentist? No. However, with a little time, scissors, and some patience, you can craft a tray that serves its whitening purpose well. I dipped my trays in hot water about 5 times per tray, each time securing it more and more by sucking the air out while holding the front with my fingers and a thumb. The first time is never accurate so give yourself a few tries and follow the instructions.Once you've made a decent impression and are happy with the fit, grab a scissors and cut around the impression of the gum line. The more precise the scissors the better. This will prevent whitening gel from becoming trapped between the tray and your gums, which can burn your gums and cause serious discomfort. Cut off a little, check the edges in the mirror, and keep cutting until you have correctly trimmed the edges of each desired tooth. Thats it! Worked great for me!