There’s a little booklet included with very brief instructions (in about 12 different languages) but if you can cook an omelette, you should be able to work out what to do. However, you should always read the instructions!
You just pull out a tray push it against the corresponding teeth (upper or lower – it tells you on the tray), suck in & push down lightly then pull of the green tray & you’re left with a thin white tray that doesn’t make you dribble everywhere.
It did make me gag a bit at first, but so does the doctor putting a wooden popsicle in my mouth but I got used to it pretty quickly & it’s really easy to use. Just fit it, watch Friends for the next 60-90 mins – no holding a little light with one hand – then brush your teeth! That’s really all there is to pearly whites.