Pull the zipper down as far as you can. Carefully cut into the right side of the zipper, making a small, horizontal cut right above the "puller". Then, thread the cut piece into the puller just like you thread the zipper on a jacket. Finally, with a needle and strong thread, sew about 7 loops over and under the place where the zipper is now threaded. This keeps the puller from being pulled down too far and unthreading the zipper. I've done this for over 30 years. It always works.