Start by cutting your pillow front to the size of your pillow form plus 1/2". I used an 18" pillow form so I cut my pillow front 18.5" x 18.5". Cut your pillow back 18.5" x 18.5"+ a few inches ( my pillow back was 18.5" x 22"). You do this to allow a little extra room for your zipper .
Now, take your back piece and cut in in half so you have two 18.5" x 11" half pieces
Leave your front piece intact :)
Press your half pieces toward the wrong side of the fabric along the 18 1/2 in. side.
(1.4 inch on one and 1 inch on the other)
Match the 1/4 inch pressed edge up close to the edge of your zipper and sew down the entire edge of the fabric using your zipper foot.
now, with your zipper attached to one side, match up the pressed 1in. fold with the edge of the zipper that is already sewn to the other "half piece" of fabric
Pin it in place very well
Flip your pinned fabric over and sew right along the edge of the other side of the zipper
(the zipper will be upside down)
Now unzip it so the pull is somewhere inside flap, then cut the ends off both sides of your zipper
Change to your zig zag foot and sew a few stitched to hold the ends in place.
Be sure your flap is down when you do this!
Now, trim the back of your pillow (the one you just made) so it matches the 18 1/2" x 18 1/2" front
Then, place them on top of each other with Right Sides Together and use a plate of a bowl to trim the edges on each corner. (This way you avoid "dog ears" on your finished cushion)
Pin the entire pillow together all the way around and be sure to keep the zipper unzipped.
Sew around the entire pillow using a quarter inch seam allowance.
unzip and flip it inside out. Add your pillow form and you're done!
P.S. You can also fill in the corners of your pillow with stuffing to make them look extra full (it's what they do when you have them custom made)