is creating the illusion of 3d objects on a flat 2d plane, like a piece of paper. On his youtube channel: Mark Chong has interesting ideas on; How to learn to draw, how to design exercises for yourself and how you develop a feeling for 3d space.
Tip for Photographing your work:
Remember that shadows cast on your paper ruin the optical illusion you are trying to create. This is also why you really shouldn't draw on paper with lines.
Slow down, don't skip steps. Get it right and speed will come with time
Make up your own exercises that show you directly where you make mistakes