To shorten a zipper from the top, first install the zipper, placing the bottom stop at the desired length. Open the zipper. Stitch a bar tack by hand or machine over the teeth on one side of the zipper in the seam allowance. Cut off the excess zipper even with the raw edge.
To shorten a zipper from the bottom, close the zipper and measure the desired length. [For an invisible zipper, add 1" (2.5 cm).] Whipstitch tightly across the coil or teeth 8 to 10 times with a double thread. Cut the zipper ½" (1.3 cm) below the stitches. Install the zipper