I used Microsoft "Word"
Word has a 'drawing' tool that can make line, circles, squares, etc. For major drawings, like Bob Nuckells', I copied them from pdf drawings using the photo copy tool (in the free Adobe reader), then used boxes of white to cover what I didn't want and added the lines I needed.
Its cheap and I didn't have to learn anything new. It's relative fast unless I did a complex drawing from scratch. Even then I could cut and paste parts of other pdf drawings. When done, I 'grouped' the drawing so it didn't shift.
It's another option, H