Pin these two sides together. If one side happens to be slightly larger then the other, no big deal. Just make sure the edges touch and let the material gap in the center. It will help with the fluffiness of your pillow.
Sew the sides together. (Just a note– Because you will be sliding your pillow in and out of this opening I triple reinforced my stitching on the opening to prevent ripping it out when fitting the pillow in.)
Pin on your next overlap side of the pillow. Make sure the two overlap side pieces do in fact overlap.
Sew the second overlap side on and you will end up with a cover that looks like this:
Turn the cover right side out.
You should have a nice little overlap that you can squish your pillow into.
I would be lying if I said it was super easy to slide my pillow in. This pillow was so fat I probably should have cut the material to be 2 inches larger then the pattern, but it still worked just fine!