Cut two pieces of square fabric the 1/2" larger than your pillow insert (ex: pillow insert is 20", cut a pair of 20.5" squares). 

Serge all four edges of each square. Trimming off as little as possible during the serging process. 

Pin the two squares together, leaving one side unpinned. 

Pin a zipper into place on the unpinned side about an inch up from the edge of the fabric.

Sew the zipper into place. 

Sew the other three sides shut, leaving a 1" margin from the edge of the fabric. Backstitch when you hit the zipper line.