This is an effective way to whiten your teeth, though you will play a greater role in ensuring the best possible results, and several weeks may be needed to achieve your desired level of whiteness. First, a mold of your teeth will be taken, and then two thin, flexible plastic mouth trays will be custom-made for you — one for the top teeth and one for the bottom. You will fill the trays with whitening gel and then position the trays over your teeth. The trays are left in usually for about an hour at a time.