i started this project just after finishing my "grass patty" stool cover... made the same way, tons of little loops, each consisting of about five chain stitches... the yarn was a second-hand cotton sweater which i unraveled and the insert was purchased at a local craft store. this plump cushion now lives on our couch with my happy cactus pillow (i couldn't resist buying one for myself!).
for the back i single crocheted into the back loop of each stitch... not following a pattern, i increased as needed, trying to keep the circle fairly flat, but knowing any lumps would smooth out when stretched over the pillow insert.
the back and front are crocheted together with a chain stitch: