Step 1: Analysis
The Curl-up (in most cases I could observe) has a carapace with 18 body segments plus its head, with the last one, the tail, finishing in a rounded point instead of attaching to another segment. There is some inner flesh that protrudes a little from the head shell, but the underside of the body is never shown clearly. There are three pairs of legs, with a quite human knee and foot attached. I decided not to worry about the soft, squishy interior until I had the carapace working. I numbered the segments starting from the one just behind the head. The legs extend from segments 4, 9, and 14. The join between segments 1-2, 2-3, and 3-4 have a significant reverse bend in them when the Curl-up is standing normally, to keep its head raised. Some of the later segments also have a slighter reverse bend but this depends on the Curl-up's movement and activity.