I will admit my hand drawing skills are a little rusty. But, how an artist thinks and processes information will never wear off.
I stumbled upon artist Marco Bucci on YouTube and wow! He has great videos for learning the basic principles of painting that apply to any form of illustration. Here are the top 3 I believe any artist should watch. He will cover how to think about light and shadows. This is basic stuff but the way he breaks it down is really informative. I hope you enjoy.
1. Merging Shapes - 10 Minutes To Better Painting - Episode
2. Ambient Occlusion (and Ambient Light) for Painters