Cut polar fleece to desired sizes. Allow for hemming: 2 to 3 inches on length and 1 to 2 inches on width.
Place pieces right sides together.
Sew sides and bottom.
Keep right sides together, roll material along top inward and sew first hem. Repeat for second hem. This will reinforce the hem. It also provides a slight separation for the animal to see that they can crawl inside.
Turn right side out and finish off threads.