I always brake it down in a few steps.
Teeth are open like saying "ah"
Lips are together normally not too far in not too far out.
Corners of your lips are firm (not Tight) like a firm handshake.
Your top lip and bottom lip stay flat against the gums and teeth.
And most importantly it is the Air that separates the lips.
I also let them know that if no buzz sound is created but we can still hear the air moving trough it then it is still good.