Since I came up with the idea and developed it, I'm assuming that one is like mine.
Measure around the rim of the potty bowl and cut the fleece about 1/2" longer than that - it doesn't have to be exact becuase fleece is stretchy. Cut it about 8.5" high. When you fold it in half, it should be about 4" high. Cut the elastic about 1.5" smaller than the elastic, you can eyeball that once you have the cozy sewn into a tube. You want two loops of elastic inside, and after you sew it closed you just push them apart and put one on top of the rim and one below. I like the strong tubular elastic for this and turtlenecks.
Or you can just buy one - I just restocked mine here