1. BB's has a round shape..everybody knows that! and start making two straight perspective line, eye and mouth line.
pic.12. 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).
pic.23. Bold the sure line, and you can erase all unused line for finishing, or smudge (coloring technique) the desirable area.