Second week
Mouthpiece practice 
And now, position of the mouthpiece on the lips. Slow! Caution! Red light ahead! This is the step that can spell success or failure. Take no chances! 

First in this step is finding the vibrating center of the lips. Vibrating center!  

You should have no trouble locating the vibrating center of your lips. Provided you have worked conscientiously this past week on the loosening process, a glance in the mirror as you buzz the lips will show the natural vibrating center. Wherever that happens to be, there is the place for your mouthpiece! 

(Embouchure  training, page 34,Prelude to Brass Playing)