Consider braiding three strings together first, and then taping them to the cardboard. This will not only make the handing string stronger, but also prettier.
Draw lightly with the pencil so that you won’t see the marks when you are done.
You can use clear tape, or patterned/washi tape.
Make sure that the rectangle is oriented horizontally, with the long edge facing you.
If your leather/suede is very dark, try a light-colored pen or a white colored pencil. Use a ruler to make your line as straight as possible.
Remember to use a ruler so that the lines will be as even as possible.
