Fig 1
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.

Fig 2  
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