To make the robo forearms for the titanfall pilot, I started by making the shape I wanted out of aluminium foil and duct tape. I have a duct tape/stuffing copy of my forearm, so I just mashed the foil on to that until I got the desired shape. I then covered that in masking tape and drew on the seams. That was cut off and trasfered to paper. It could then be assembled in foam.
Some tweaks to the pattern need to be made but it's a decent start!