I have used Microsoft paint to draw knots, but it was clunky and time consuming to produce a drawing of what I was trying to describe. I currently use AutoCAD, which I use at work, but at over $1000 a seat, this is not really an inexpensive or easy solution. There are several low cost and free drawing programs, and most of them are intuitive, so should be easy to learn.
Another option would be to take photos of each stage on tying a knot. Paracord comes in many different colors and should photograph well.
Good luck! Pat