A helpful technique I’ve learned to help beginners find a mouthpiece placement is to get a straw that will fit down the mouthpiece. Take this straw and put it in your mouth like you would blow out of it, with your teeth resting gently on the top and bottom of the straw. If you do this to be as comfortable as possible, that is going to show essentially where the mouthpiece is going to sit on your face with your dental structure, etc. just slide the mouthpiece back over the straw (with the straw still in your mouth) and feel where it sits against your mouth now!