I know that i am not a pro, i am a suck artist, but i think i have realized something about still life drawings, forgive me if i done wrong, i hope these normal tips will help u!
1) use black spray for making the last step shadow
2) use at least three color steps for shadowing
3) make sure that the color steps have been mixed
4) first do your drawing then shade crazy, after all give complete shape by white limited fill
5) use the layers for tricks
6) don't out line any still life drawing, if u must do, out line with the color what you are going to apply in the center.