Feel Teeth Behind The Lips
The advice here is pretty simple. I aim to anchor the mouthpiece on my face in basically the same place every time I put the horn on my chops. I generally anchor from the bottom, meaning that the mouthpiece pressure on the bottom lip is greater than that on the upper lip. I apply just enough pressure that I can feel my bottom row of teeth pressed against the inside of my bottom lip. It's a rather gentle thing. Don't go slamming your mouthpiece into your face!