Normally I start with the head, like most people, and do a general outline of the rest of the body's proportions. Then I sketch more lines over the body, kind of 'sculpting' the shape I want, and eliminating the unwanted lines in the end. However, occasionally I will start with something like the legs/rear, if the legs are in a bent/raised position. I find it's so much easier to do those parts first in that instance, since they are the hardest part of that kind of shot (and then work my way up from torso to head). This is quite difficult to do, but it's far worse for me creating a great upper body, and then struggling with the legs. Also, sometimes it works well to do the legs first if you want the legs/feet entirely in-frame. Sometimes, I do the upper body first, and then won't have enough room for the lower body, as I underestimate how long the legs really are.