Diagrams are drawn with a Pilot Fineliner (greatest pen ever) in an artist's sketchbook. I then take a photo with my phone, crop to size, and run the image through a two stage conversion process. First ImageMagick converts the image to grayscale and cranks up the contrast. Second, Potrace converts the grayscale bitmap to an SVG. This was my hack way of avoiding the purchase of a tablet.
One interesting consequence of this process is that every drawing needs to be perfect the first time. I didn't realize how much I lean on undo/redo until it wasn't there any more!
