OK, be kind in your comments if you already knows this. However, I've never seen this technique in a book and a search didn't produce a tip like this on this site so here goes:
THE EASIEST WAY TO CENTER YOUR ZIPPER
1. Baste the seam in which the zipper will be placed.
2. Open your zipper
3. Place the teeth of the opened zipper (one side only) right on the seam line (wrong side of seam to right side of zipper.
4. Using a straight stitch foot if you have one or a zipper foot if you don't, baste this size of the zipper to the seam, stitching as close to the outer edge of the zipper tape as possible. Don't pin or otherwise secure the zipper to the garment at this stage. Just use your eyes and hands to keep the zipper teeth on the seam line.
5. Close the zipper.
6. From the right side stitch the zipper to the garment using a zipper foot or another foot of your choice. What ever foot you have needs to have a guide you can place along the seam line. Start sewing on the side that has the basting and then sew the other side. Note that you have only had to baste one side of the zipper before you sew it to the garment.
7. Rip out basting stitches on the zipper and the seam.