There's a nice demonstration/directions on you tube. I've made them just by measuring a pillow case; adding inches for the top. the bottom I increased it by 12 inches and cut the piece in two pieces. One 6 inches longer than the other. Then folded one end of each piece in 3 inches after I made a one inch hem. Then I matched the front with the two back pieces - the back pieces will overlap. Sew all around the edges and turn it inside right :)
The video explains it better :)