First cut your backing fabric as listed in the chart below:
14” pillow: 5.5” x 14.5” and 11.5” x 14.5
16” pillow: 7” x 16.5” and 13” x 16.5”
18” pillow: 8.5” x 18.5” and 14.5” x 18.5”
20” pillow: 10” x 20.5” and 16”x 20.5”
Use a zipper that is longer than the pillow size – it is much easier to sew if the zipper pull is completely out of the way. Interface, apply fusible fleece, or use batting and lightly quilt the pillow back fabric pieces. In the photos, the red fabric is the right side (Photo 1) and the white is the interfacing already applied to the wrong side (Photo 2).
Fold 1 3/4” to the wrong side of fabric and press (Photo 3).
Lay zipper face down, with pull to the right, along the edge of right side of the fabric on the edge with the pressed crease (Photo 4). Use clips to hold the zipper.
Using a zipper foot, sew the zipper down – still with the zipper face down. This seam will be on the back/wrong side of the zipper. (Photo 5) Then sew along the crease. (Photo 6). Do not top stitch.
Fold along crease, bringing zipper to the back of fabric. Look closely at Photo 7 – blue arrow is pointing at the crease. Note the zipper is facing right side up. Still with a zipper foot, sew down the zipper – this will be through several layers including the zipper tape, and both layers of the flap. Black arrow is pointing at the seam line.
Photo 8 shows the zipper with flap attached.
Lay the remaining half of the pillow back right side up. Lay the half with the zipper attached right side down on top, aligning edges. The two pieces are now right sides together. Unzip the zipper and clip the free side to the top edge. (Photo 9) Again using a zipper foot, sew down the zipper (Photo 10).
Photo 11 shows the zipper closed and completely sewn in.
Open the zipper into the body of the pillow back, and trim both ends of the zipper tape even with the pillow back (Photo 12). Be careful not to accidentally pull the zipper pull off the zipper on either end, as now there are no stops on the zipper tape, they have been trimmed off.
Lay the pillow front right side up. Lay the completed back face down aligning the edges on all four sides, trimming as needed so the front and back are the same size (Photo 13). Open the zipper at least half way so that the pillow cover can be turned right side out after sewing the front and back together (Photo 14).
Pin or use Wonder Clips around all four sides (Photo 15) and double check that the zipper is open (Photo 16) then sew around the pillow cover using a 1/4” seam allowance (Photo 17). Trim the corners (Photo 18).
All done! Photos 19 and 20 show the finished zipper under its nice flap looking very professional.