First I pinned strips of foam to my dummy, getting the shape I wanted. I drew lines to use as a guide for when I hot-glued it together.
After gluing the main part, I added the collar and waistband, then the ties to hold it on.
I used dabs of hot glue for "rivets," primed it white, and painted the whole thing with several coats of metallic paint. I used gunmetal, copper and gold in successive coats, to build up the color I wanted. Then I did one last wash of watered-down gunmetal and wiped it off again, for a weathered effect.
...Now I just need to line it with fabric, and make the entire rest of the costume! Woo.