Quoting Finn C-Q
How does that elastic technique work?
I tired to explain in an earlier comment, but pretty much what you do is thread a white LEGO rubber band through the bottom of the torso, up through the empty arm-hole (this may take a few tries). Then thread this rubber band end through the lever cap piece, and around the arm socket. You can look at the pictures for a visual.
Thanks for all the comments everyone! Feel free to use the arm technique (credit appreciated), all you need is a white LEGO rubber band and one of those white caps. Just string it through with the cap to give it more of a curve.