Long tutorial. This used to be called a Lip Tutorial but I changed it midway through the text and forgot to edit half of it. But same concept throughout.
Currently Available Tutorials:
Eye Coloring and Lineart
Gesture and Curvature
Mouth Anatomy - Part 1: Location, Shape, Angles, Curves, and Easy Movement.
Remember the disclaimer: This is stylized realistic, my own preferences. But the methods can be used with almost any style and preference.