For the studies I paint with two values (light grey for light and dark grey for shadows). Using my reference image, I use the navigator or zoom out of my canvas and simplify what I see into two values and paint that.
The point of the studies is to make myself not zoom in and rely on seeing the bigger picture to paint the values. It's a problem that most people have where zooming in causes you to paint something that might not contribute to the overall painting. I don't make a drawing underneath (optional) since I'm essentially sculpting and having a guide would be a hindrance to the process. Though I did draw a bit for the Zeus study since his beard is so complex and didn't have the knowledge to render that.
For a simpler explanation, look at Sinix Design's beginner guide on digital painting on Youtube. He explains alot of the abstract ideas when he paints and how he suggest beginners to paint with the hard round brush: https://www.youtube.com/watch?v=-Nt9fa8jZUE&t=438s
Even though it might not be in your range of vision, doing studies of real life objects and painting similar to the video is extremely beneficial to your art career and can help with many of the abstract fundamentals Marc describes in his course.
If you have any more questions please fire away.