How to Replace a Broken Zipper
How to shorten a zipper
You may need to shorten the new zipper to fit into the fly opening. To do so, measure the length of the teeth portion of the original zipper, and measure down from the top stop of the new zipper. Stitch a new bottom stop by zigzagging across the zipper teeth; then cut off the bottom of the zipper with diagonal cutters. Note: It's not advisable to shorten this zipper from the top, because, in this repair, the upper end is not enclosed by the waistband. Without an upper stop, the slider is likely to slide right off the top of the zipper, leaving you with one option for getting out of your jeans-scissors!