I used the pattern from Camille's book Simplify. It was easy peasy, but I would do the envelope back a little differently next time. I would definitely allow more fabric for the foldover. Have made a note on the pattern.
Then I made a dozen of these....fun pillowcases and the eight grandkids took their pick....and a couple adults got the rest! Very fun simple pattern. It is called the 20 minute pillowcase and it encases all of the seams....sweet! I don't remember where I got the pattern, but if you google the name, I bet you will find it.