Here's a quick and easy way to insert an invisible zipper. First, unzip your zipper and place face down on the right side of your garment like so:
Using a zipper foot, sew a line of stitches as close to the zipper teeth as you can get without actually touching them with the needle.
Once you have one side down, pin the other side in the same manner (zipper face down on right side of fabric.) Before sewing, zip your zipper to test that it closes properly. If it is twisted, correct and re-pin. Use the zipper foot and secure with a line of stitches. Be sure to backstitch to reinforce your sewing.
You're almost done! If your zipper is longer than the opening you wish to create, you can trim it to your desired length and put a horizontal line of stitches across your zipper to create your own zipper stop. Be sure to leave some extra zipper length below the zipper stop (about an inch).
Now, using a standard foot, close up the rest of the seam below the zipper being sure to continue your stitches all the way to where the zipper ends to make sure there are no holes. Press seam open.
Zipper insertion is now complete!