Step 5: Add the trim
This is where your own creativity comes into play. Besides looking good, the thing your trim is best at is covering up glue lines. When adding the trim, try to glue the edges, but use a small bead of hot glue to reduce glue seepage from the edges (a little can be okay though, and could be incorporated into the weathering effect later).
The first thing I did was cut out 7/8-inch thick strips of black 3mm craft foam. I carefully measured around the helm, and added roughly an inch to each length to account for error. Cutting these is easiest with a rotary cutting tool, cutting board and straight edge, but it can be done with a hobby knife rather than rotary cutting tool. A big paper cutter would probably be even better.
You will find times when you need to decide whether to cut the segment to fit or to overlap. From my experience so far, I would recommend taking the extra time to cut the strip that would overlap another, because in my opinion I think it looks a bit better. I recommend gluing the strips where you need them first, then carefully cutting them on the helm itself to fit. The heat from the hot glue tends to warm up the foam to a point where it stretches easily, and your dimensions will not be as accurate as they were on the cutting board.
Using the stencil from step 4, I cut a second face plate (black, pictured above) out of 3mm craft foam. This piece may not be necessary, but I liked the way it looked at the time. Be sure to use a lot of tape, and follow the same procedure as with the thick face plate. I cut the eyes out slightly larger so that they showed some of the material behind it. Glue the black face plate on with hot glue being careful to line everything up properly.