This is an optional step, but I think it greatly enhances the finished project. You'll need a light piece of cardboard and a compass. Set the compass to a radius of about 2.5 inches and draw the circle.
I used a soy milk carton for my cardboard. You could also use cereal boxes, thick mailers, notebook covers, etc. Just find something lying around. :)
Once you have the circle drawn and cut out, fold it in half. You'll be able to fit in through the opening this way. Once it's through the opening, make sure to open it fold side up. This way the stuffing will keep it from shifting about.