if no one explain it to you, I'll show it to you tomorrow, my time. Have to go to bed, long work day tomorrow.
But I can give you one example (that other people can use to explain things with):
Shipbucket have some drawings rule, one of them is that if a surface is angled from you'r perspective, it should be marked with a very dark grey color, and the surface with a color darker than the original grey used. The drawing above show that on multiple ways (It doesn't mean you have to draw like that drawing, it's a little to advance to go for that level of detail, this early, but later when you have learned all tricks in paint.) (the Drawing above is drawn in paint)