|After a failed attempt at inserting a zipper into a spandex/lycra wetsuit, and much research forthwith:
Cut out two 3/4 inch strips (zipper length) of fusible interfacing, and iron onto the wrong side of the zipper opening. Baste the zip in place then sew with sewing machine.
(I used the interfacing that comes in a bag at Walmart, not sure what brand)
After ripping out my rippled zipper, I'm really happy with the smooth results!