I do basically the same but if it's just a casual throw pillow I use 1 piece of material. Cut it wide enough to cover the pillow form and long enough to go all the way around the pillow plus 8 inches or so. I hem the two short ends, then wrong side out I fold the two ends to the middle, overlapping them about 4 inches, then just sew the two sides together.
Buttons are good for holding it closed too.