$1 JoAnn Fabric Swatch Sale: Make New Pillow Covers For $5
I picked this pretty and modern print and a plain white for the back. Place them on top of each other and even up the ends if needed. The swatches are roughly 17". You should pre-wash too since the beauty of zipped pillow covers is that you can remove them!
Place the zipper along one edge, right sides together, and sew with your zipper foot.
Now sew the other end of the zipper to the other piece of fabric (right sides together). Open the zipper because this will be your opening for turning the case right side out later.
Sew around all 4 sides, clip the corners and turn. Voilà!