Create a Thread Stop
Before inserting a zipper into a zipper tab, use a sharp hand needle and thread to whipstitch the two sides of the zipper tape together at each end, right over the zipper teeth. (You can also use the zigzag stitch on your sewing machine, being careful that the needle does not hit the zipper teeth.) This will make the zipper easier to work with. Keep the stitches snug, as if the thread were making a new stop for the zipper slider.
Shorten a Too-Long Zipper
With the zipper closed, measure the length you need and use thread to create a new stop, as described above. Then simply cut off the excess length. If you find yourself making many zippered projects, it can be cost-effective to buy long zippers in bulk because they can be shortened to fit many projects.
Go Slow and Pin a Lot, Especially Around Curves
Keep the zipper teeth against the zipper foot of your sewing machine to ensure an even seam. You should be able to feel the zipper under your left-hand fingers as you guide the fabric through the sewing machine. If you find your seam is getting off or the zipper tape is sliding out of place as you sew, stop stitching with the needle in the down position, lift the presser foot, and adjust the fabric and zipper tape back into position. You can do this as many times as you need.
Don't Stitch Over Anything Metal or Thick Plastic
Know what kind of zipper you are using before you start. If it is a polyester or nylon coil zipper, most sewing machines can stitch through the zipper teeth (but not the metal stops) at a slow speed. However, stitching over metal and plastic zipper teeth and sliders can break machine needles. Make sure you know what the needle is about to hit as you sew. If the zipper slider is getting in your way, take the project out of the sewing machine, pull the zipper slider a few inches up to where you've already sewn, and then continue sewing with it safely out of the way.