I like Dan Callaway on YouTube. Jeff Rolka is another good teacher. However, singing is complicated and choosing which exercises you'll use to teach folks ... You need 1. Vowel alignment 2. Coordination of voices 3. Breath support and vocal tension management 4. Mouth shape, tongue placement.

And then there's "how to read a song", deciding which words are emphasized, where to breathe, modifications to fit your voice, etc.