This is by far my favorite technique for making pillow covers. You don't have to sew a zipper in and the cover is still removable so you can wash or change them. I came across this pattern on Lovesome's blog and can't believe I didn't think of it before.
You can follow the measurements on Lovesome's blog or lay your pillow down and eyeball it like I did. I put my pillow down on my fabric, wrapped the fabric on the right side over the pillow, so that it came about 1-1/2" past the center. Then I did the same with the left side, with the fabric overlapping about 3" in the center. Make sure there is about an inch of extra fabric along the top and bottom of the pillow that can be sewn together.
I then pressed and hemmed the side edges of the fabric so that I had a clean edge.
Lay your fabric down with the right side facing up. Place your pillow in the center and fold the two sides over, pinning the center. Make sure the fabric is snug against the pillow and smooth. I then pinched the top and bottom layer together along the bottom of the pillow and placed a pin in the top layer to show where my seam would be. Repeat for the top of the pillow.
Pull your pillow out, lay your fabric flat and put a couple more pins along the top and bottom seams. Sew the seams along the top and bottom, using your pins as a guideline as to how big your seam allowance should be. Use a serger to finish off the edges or zig-zag stitch along the edge and trim the excess material.
Turn your fabric right side out, iron the seams and any wrinkles and stuff your pillow in there!