Cut a piece of cord 6-8" and feed through the zipper.
Hold the two halfs of the cord together making one thicker cord.
Loosely tie a "Handover" Knot with the two cords together.
With the knot loosely tied you can slide the knot up or down to adjust the loop size.
Once your happy with the size of the loop then tighten the knot.
Next cut off the excess cord from the loop about 1/4" below the knot.
Last, with a lighter carefully touch a flame to the cut tips of the loop to melt the fuzzy ends of the nylon.
This will keep the cord from fraying and falling apart.