This method to shorten the zipper is intended for projects that do not require fabric tabs at both ends of zipper.
1. Measure the desired gross length of the zipper for your project.
2. Mark 2 lines at the both ends of the zipper, with 1 represents the actual length while the other line represents the seam allowance (click to enlarge photo below)
3. Make sure the zipper puller is within the area you'll use and won't be cut off.
4. Using zigzag stitch (or hand stitch) at the open end of the zipper, sew a few lines within the seam allowance to close the zipper end.
5. Trim the excess zipper (click to enlarge photo below)