Start by measuring your own original cushion cover that you want to replace. If you don't have it just measure your pillow and add maybe 1/4"-1/2" ease. Mine was a 15.5" square. Add seam allowance of 0.5" x 2 = 1".
Cut your fabric to your measurement. Mine was 16.5" square. Oh I have a terrible habit of not pressing my fabric before I cut...as you can see in the above picture...but don't do that...press your fabric first k :)
Pin one of the seams (this seam is for the zip). Machine baste (use the largest stitch length setting on your machine) this seam with a 0.5" seam allowance.
Then stitch 2" from the 2 edges of the seam with a regular stitch length to reinforce. I stitched up to 2" because my zip was 15" long. Decide on how far you need to stitch up depending on your cushion size and your zip size.
Pin your zipper to the center of your seam. Look at the above picture. This is what I mean by 'decide on how far you need to stitch up' . The end of my zipper is about 1.5" from the edge of the seam. So I decided to sew up to 2" in to encase the zipper.
Using a seam ripper, rip open the machine basted seam. Leave the 2" (that you reinforced ) of both edges closed.