AH is the most open of all the vowel sounds – your jaw is open, your lips are open, and the back of your mouth is open. Rest the tip of your tongue against the backs of your bottom front teeth and gently open your jaw.
Relax your lips. Your tongue will pull back a little and you will feel the sound in the back of your mouth.
Try yawning on the sound and feel how open you can stretch your jaw and the back of your mouth and the top of your throat.