Someone wanted to learn this too, so Grasp built them a personal learning path.
You can keep exploring this one or create a path like this one shaped around you.
Recommended resources
Module 2
Creating Form: Light and Shadow
Module 3
Constructing the Head: Front View
Module 4
Drawing Expressive Eyes
Module 5
Drawing the Nose and Mouth
Module 6
Constructing the Head: Profile View
Module 7
Constructing the Head: Three-Quarter View
Module 8
Adding Life: Hair and Basic Expressions
Module 9
The Complete Portrait: Observation and Block-in
Module 10