I didn't use a pattern, rather I took a length of fabric about 5' x 1.5' (yes that's five foot because you sew the short parts just like on a pillow, and then wrong fabric sides together sew up the sides. If I was making a real pillow case I would have made it bigger more like 6' X 2', but you could measure your pillow to be sure.
I sewed the applique before I sewed the case together (I just measured up about 5"-10" and worked in that space) so that the backside (ugly) part of my work wouldn't show, but you wouldn't have to do that.
I wanted the pink ribbon from one of my daughter's dressed to mark off the applique area so that had to be pinned on before I sewed the "tab". If that's not as clear as mud I looked up a free pillowcase making website for you here.
http://www.alternative%2Dwindows.com/pillow%2Dcases.htm That's just one if you google pillow case pattern free, you can find lots of other more complex patterns.
It might not sound like it but it was easy! If you have the fabric besure to give it a try! :D