Unlike normal nylon zipper, you can’t cut and sew through metal zipper to shorten the length. Metal zipper has to be shortened by removing the excessive metal teeth so that you can cut the extra length before sewing to your project. With the correct length, you don’t have to worry that you might accidentally hit the metal teeth and break the needle when sewing across.
I wish I have all the colors and sizes of metal zippers stock in my craft room or….. I stay next to a zipper store. In reality, I am not and I only stock a few for some lengths (4″, 8″, 14″ & 18″) in some most popular colors (white, beige, brown & green). I used this method to shorten all my metal zipper sewing projects, like this, this, & this… and many more to come.