Well, I do all with mouse and lines, first I always use semitransparent lines with vibrant colors, and make them cross, then I delete the excedent, later when I have all lines finished, I make the inside, and at the end I put the colors
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That's and old drawing someone give to me, here is the original one, and the digitalization I made:
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Things like magic, semitransparent objects, eyes, shadows and gradients are more complex stuff, the lines it's just the basic