The key improving nasality is control over the soft palate. This affects how much air moves through the nasal cavity in addition to the air moving through the mouth.
Watch the soft palate raise every time you say the open “ah” vowel
Lip trills Note: Lip trills cannot be done successfully unless all the air is coming through the mouth
Push up on upper lip
Exercise Examples
Octave slide in front of a mirror
Slowly speak each vowel taking time to make sure the palate is lifted (Disney Voice)
Inhale on a “K” sound
Glottal stops on “Ah”
Simple exercises with “B” consonants ex. “Bi Be Ba Bo Bu.” Make it easy so that they can focus only on singing each vowel without nasality