The way I do..sew 2 squares together on only one side, but just at the ends about 2 inches..leaving open between. I then like to baste the remainder of the seam with lge stitching. Install the zipper and open slide (for turning later. Then I sew, wrong sides together, the remaining 3 seams. It gets a little more complicated when there's cording, fringe. or ruffle. But this is quick and simple. If you are are using a 'truly 'invisible' zipper, then the seams are sewn after zipper is in.