1. BB's has a round shape..everybody knows that! and start making two straight perspective line, eye and mouth line.
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2. Imagine a head wearing Mich helmet and make a smooth gap line between the base round line. dont forget to add some details (tips: sketching technique by using fine pencil do not bold the line before the finishing process).
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3. Bold the sure line, and you can erase all unused line for finishing, or smudge (coloring technique) the desirable area.