Use this method for both the upper and bottom trays.
1) Fill a pan with 2" to 3" of water and heat to boiling.
2) Remove the pan from the heat source. Holding the tray by the tab, dip the tray into the water for 8-10 seconds. WARNING: Allow water to cool for a few moments before dipping tray. The tray becomes very flexible at this point and can collapse if water is to hot.
3) Remove the tray from the water. Wait three seconds and insert the tray into your mouth. WARNING: Be Sure the tray is not too hot for your mouth. Test with finger. While gently biting down, suck the air out of the tray until the tray fits tightly around your teeth. Press with tongue, lips and fingers to form tray.
4) Wait 15-30 seconds and remove.
5) Rinse the tray and trim away the tab. For a better fit, you may also trim the upper edges of the tray to more closely match the contour of your gum line (this especially helps for those with smaller mouths)