I think it should be pretty simple if you get some stretchy, hose-like material, cut some plastic sheeting or foam into points, and stuff the hose with those shapes. then, for the antennae, just stitch them to the top of the suit. For the hands, likewise. Plastic sheeting can easily be cut and glued into points for the fingers, which could be put into stretchy gloves or something similar. Same with feet, I would think.
Anyway, best of luck!