I start with a picture or image I think best represents the subject. I tool around with it until I am satisfied and then transfer that to the canvas (I use an old technique I learned a long time ago without much help, just a pencil and ruler).

I transfer the images to the canvas as pencil, then I go back and draw the lines with a Sharpie pen. I erase all pencil lines, since the lead will get mixed with the medium and get all greyish and dirty.