Surprisingly I have never had to tie a string heddle before but I made a drafting mistake with my Crackle shawls. I need to add six threads but with 852 threads I am not going to re-thread the entire warp!
So to start you need a strong piece of thread, I am using pink 2/8 cotton. You don’t have to use pink, but it sure is pretty! You will need a piece that is double the height for your harness plus a couple of inches for the take up from the knots.
Last step is to go around the top of the heddle and tie two knots to hold the string heddle in place. Make sure that the string heddle is tied not too loose or too tight; you want it to slide across the harness but not to be sagging.