Finish your studies that focus on shadows. When adding shadows from imagination, use logic and mentally calculate what will be illuminated or in the shadow. If you're after accuracy, you can recreate the scene in real life with the help of friends and family. Rather than guessing where the shadows would be, you would be able to see it.
Continue exploring how to use washes to express texture and color (light vs dark colors). Use either different pre-made toned washes and/or layer your washes.
The Seven Wonders of Washes:
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