Step 1: Dummy
I first built a duct tape seiwng dummy by having my wife and daughter wrap duct tape around my torso. then we cut it up the back and stuffed it. I mounted it and had a good dummy on which to build the costume. The picture below shows a long sleeve shirt (costume inner liner) surrounded by a plastic frame I built out my my kid's K'Nex pieces. Worked better than expected. On this I mounted 1/4" carpet padding foam and then I stuffed poly fil under that. Wrapped the whole thing in nylon and sewed away.