shading is easy when you know how shades and light work ,,,you can observe shadows everywhere, i didnt know it before but now i can tell if i image is manipulated or not cause of the shade position,s the best way to shade is just with a cube or a egg , example this
The best thing to do is observe from life - get a sphere-oid and put a hard light on it. Move the light around, etc.
Visual guides like Ban Kai's will help you figure out what you're looking at (I'm studying out of Bridgman's anatomy to help me wrangle some sense out of my life studies), but in the end, you really should draw from life.
Think of it this way - would you trust an English translation of a German translation of an Italian proverb copied from a Chinese textbook? Go to the source.
In painting/drawing classes they sit you down in front of some white boxes, cups, cones and eggs with a strong side-light and have you copy them. I'm sure you can grab a desk lamp, find some simple white objects around your house and start practicing. There isn't some sort of crazy secret to it. Just put some things together, shine a light on them and keep drawing until you can make it look good.
Usually the source of light creates reflected light, which for convenience can be treated like independent light sources. If it's a sphere on a flat plane lit by sun, there would be a direct sun light, flat plane of diffused light and diffused light hemisphere of a sky if there is an atmosphere.
Also dividing objects into basic geometry forms and building their intersection is a valuable skill. just look for info like that on the net.