First attempt to draw such patch effect drawing

I am a pencil artist who is going to teach students drawing, but I don't know where to start. I found Proko's basic drawing gives me some ideas about teaching drawing basics, so I take this class for my reference to teaching drawing. Here is my simplified portrait drawing.

I used the Monster clay hard to make a Donald Trump pancake sculpture. For the smooth finish, I used combination techniques of thinner liquid and torch to melt the surface a little bit and smooth it.

I gained drawing and ceramic art techniques in China Academy of Art for my undergraduate studies, top-tier art academy in China, but what I was trained is in China's approach which was developed from Russia. Now I would like to study the drawing basics again through the authentic Western approach. I post some of my drawings here. Any critiques are welcome!