a pillow form (mine was 18"x18")
2 pieces of fabric cut 1" bigger than your pillow (mine was cut to 19"x19")
1 zipper - 4" smaller than your pillow (mine was 14")
Start by placing your fabrics right sides together. Find the top middle of your fabric and line that up with the middle of your zipper. Place 2 pins in your fabric to mark either end of your zipper (one at the "slider" and one at the "stopper"). You'll use these pins as guides.
Using a 1/2" seam allowance, sew from pin to outside corner (on both sides), backstitching to secure your stitches.
You'll now have a big gap in the middle between where your 2 pins were. Run a basting stitch from "pin" to "pin" (use a long stitch length because you'll be ripping this out in a few steps). Your 2 pieces of fabric will now be sewn together at what will become the bottom of your pillow.
Next, with right sides of fabric apart and face down on your ironing board, iron your newly created seam flat (like shown in the picture below).
Place your zipper on top of this seam (face down so the zipper part is pressed against your basted seam). Using a zipper foot, sew in place.
Once you have sewn around the perimeter of your zipper, flip your fabric over (to the right side) and with your seam ripper, remove only your basted stitch. This will expose your newly created sewn-in zipper.
Now onto the easy part! Pin and stitch around the remaining 3 sides of your pillow (with wrong sides together) IMPORTANT: make sure you unzip your zipper a little bit beforehand so you can turn your pillow right side out.
Clip your corners and turn right side out. Place your pillow form in and enjoy your new pillow (with zipper)! If you're anything like me... you'll go searching through the house for other pillows to cover.