Hope these help. I use a piece of fleece 11 inches by 19 inches and the 19 has to be on the stretch, when you pull on the fleece it will stretch more one way than the other.
fold in half and stitch the end together.
turn right side out and it is finished. You can ether leave the edges alone, wavy cut them, or hem them, I found if I hem them they like them better but be sure to use a stitch that will stretch.
Again Hope this helps.