>>308964098
I'd recommend trial and error. If there's 12 bones (with 12 o'clock being at your dick), you'll probably want 1 and 2 o'clock for the right legs and 10 and 11 o'clock for the left legs. Depending on the length of the skirt and how the bones move it'll be the first row directly parented to the hips. In this example it'll be four constraints in total.
Try it out and see how it moves.