I taped the templates for the main cup, left arm, and right arm together and then made that a single piece. The arms overlapped the main cup quite a bit for me when I taped them together. You can adjust that based on how much of an arm you want them to have, usually just so the arms touch in the center will work.
They arcs will become the lower portion that scoops back under the kneecap, I hotglued and bondo'd these to the main cup. The extras for the arms were similarly attached.