Cut your zipper length about 1-2" from pillow plate corners.You can use #4.5 chain zipper (same as for cushions) or 2.5 chain zipper which is a lighter weight for throw pillows etc. Put the slide on the proper end. Fold a piece of pillow fabric and sew far enough back across it to leave a small pocket to hide the zipper slide when the pillow is closed. Sew another fabric tab across the other end of the zipper.
Skip this step if your pillow has no welt. Center zipper piece to pillow plates. Start sewing at the plate's corner and the center of the fabric tab, expanding outward as you approach the zipper. Then sew as close as you can to both the zipper teeth and the welt cord.
Next fold fabric 1/2" and top stitch the other pillow plate to the zipper. Try to cover at least half the zipper teeth. You shouldn't be able to see any of the zipper at this point. If you don't have welt cord on your project, sew both plates in the same manner. Back stitch or sew twice at the beginning and ending corners as you learned in "Cutting and Sewing Basics".
Try to use a thread color that blends with your fabric. The better you
can hide the zipper the more you'll avoid the "why didn't you use a colored
zipper?" complaint. I used black thread so YOU could see it.
More about zipper instalation and how to make a boxed cushion.