I came across this wonderful pattern at the Sew Thankful website. Tracy Chapman, the owner and designer, produces wonderful patterns for free projects.
As someone who's in too much of a hurry to figure out my own patterns, I really appreciate having all the directions laid out in good order for me. This particular free patterns has the detailed math formula included along with pictures of important steps, and a host of ideas for using these very handy pillow covers.
The file is in .pdf format so you can save it to your computer.
Tracy has generously given permission for this pattern to be used for personal and non-profit projects. She also permits use for guild and group projects, or for individual crafters making their own small quantities to sell at craft fairs.