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Thank you for your comment! I see you've done some shading already, so maybe you already saw my super basic tutorial. Shading is really a learning process that has taken me a long time to get comfortable with. Sometimes I'll use one size of brush and then another over it. Also, for trees, I do lines to represent bark (if you look close, though, it looks like I scribble). Don't be afraid to go crazy with the brush. I just soft brush over with the color if I think I went overboard on shading. Ugh, maybe there are more professional artists out there who could explain it better? I watched people's playbacks of how they draw to learn a lot too, so try that. I hope that helps at least a little!