Once you have the knack of opening up the jaw when singing high, experiment with the vowel sounds and the shape of the mouth. For many people, a wide rather than a long shape works best (try to keep the lips as relaxed as possible).

Take a song that you’re working on with a difficult high note, for example a word with an “a” vowel (“take”, “make” etc). Starting low in your range, sing gradually up the scale, changing the vowel to a rounder shape as you sing higher and higher, i.e. to an “eu” or “o” vowel. At the same time relax and open the jaw, focusing the note in the mouth. Putting a “g” in front of the vowel can also help “ga, ga, ga, geu, geu, go, go” etc.

Also, experiment with the larynx. It can help to imagine that you have a ligament within the larynx that’s moving, but the larynx itself is staying relatively still, making the changes the larynx is having to make to find the tuning of the song much more subtle.