What fabric is it? That can make a difference as to whethr or not you need to stabilise, but mostly I don't on the hems of full skirts.
Hang it for a day or so (on the dressform) to allow it to drop if it wants to, and then measure. I do it from the waist seam down and stick pins parallel to the ground every couple of inches. It takes a while to do, but if the pins are close enough together it gives you the cutting line. The more curved the hem the smaller the turning should be. A full circle skirt only needs about 1cm at most; any more and it will end up with pleats rather than being eased in.