I first wrapped my rulers in tape so that the Sharpie wouldn't get on it. Then I wrapped the string around and taped down the ends.
I then made lines down the string. Make sure you go down and back up with the Sharpie so you can get greater coverage. Also, work from left to right or top to bottom if you are right handed. The Sharpie rubbed off on my hand.
Then I let it dry for a minute or two. I removed the string and wrapped it around my spool. I plan I making several more in different colors. I think they make a cool addition to any crafting area you have, plus they will come in handy for a future crafting project!