Self-Portrait, 13 x 20″, really thick acrylic.
Photoshop: Image > Adjustments > Posterize (At least 7 values)
Grid transfer to enlarge the drawing
Separate, draw, and number different values (1=lightest, 12=darkest)
Paint with monochromatic acrylic
It’s a pretty simple lesson, geared toward a quality end product, and it has usually been very successful for my students.
Note: If you don’t have Photoshop, sign up for an account at photoshop.com. You can Edit > Sketch your images, and get a very similar effect that will still work.