Hi, I've created this tool that you can use to teach yourself shading.
It automatically generates a primitive in the chosen lighting situation with the chosen material (plastic, metal, clay), then gives you the position of the light source to let you train your ability to visualize lighting and then put it down onto a canvas. It gives you grades — overall and in the different sections (core, highlight, etc) — and allows you to analyze how you did.
The tool also has a live, automatic improvement graph, and it has a cyclical timer which allows you to quickly pick the time and get into the rhythm.
In the hundred hours I've spent programming and testing this tool, I've definitely improved and I can visualize shading un-shaded objects much better.
I'm happy to answer questions if you have any.