Thanks! I actually just draw these directly into Photoshop, using the pencil tool (which is the same as the brush tool except that it has hard edges). I don't have any other special tools or processes -- I just zoom in really close and draw. Sometimes I use reference and sometimes I'm drawing out of my head (in this case, I was actually using my own life drawings as reference). There's a lot of nudging and redrawing as I try to get the shapes right, but otherwise my process is just plain old-fashioned observational drawing.