actually all has the same theory. see the plastic part on the left, and if it's not that kind, it would be metal one or another kind of plastic one, you just put the mannequin upper body vertically to the legs part, and make sure the plastic part or the metal part go into the hole correctly, then turn the body to the front.
how to make sure the plastic or the metal fitting into the hole correctly> some times you would hear a voice, and generally if it's correct, the body will be stabely on the leg port, you could turn the body easily to the front. before you turn the body to the front, just make sure that the fittings go right.
if it's small plastic fitting with a triangle cap, you don't need turn the mannequin, just put the body on the leg forms directly.