This technique brought you such great props as Bucket Helm, Scary Black Knight Helm (now peeling) and Big Giant Skull Heid (TM). Essentially the same techniques were used for the Nest's little shields that came into use in April 2006: two layers of thick camping-mat with hot glue, papier mache and some sort of paint. Contact adhesive is better than hot glue for large areas though.
What you need:
First off, cut out the shapes you need out of carry mats. This is the hard bit. It will definitely save you money to cut the shapes out of cardboard first to try them on before wasting your carrymat.
The shapes you need are roughly like this.
As long as you are careful when measuring and cutting, this one's easy! Just make sure that you glue the edges exactly, so the whole thing is like a cylinder.
To paper mache: Mix PVA glue and water half and half in a bowl. Dip or soak your newspaper pieces in the mixture one at once and simply cover all of the carrymat. The first layer is trickiest to get to stick, so paint the carrymat with pure glue first, then contine for at least 4 layers of paper, letting each layer dry in between. The more layers, the stronger the helm.
Decoration: You can add decorations by cutting out bits of cardboard into shapes and then papering only a couple of layers over them. They will be fairly bold though, paper mache is difficult to get detail with. Use your glue gun to make little blobs that will look like rivets.
Painting: (At this stage, you might want to latex the helm, but if not here's the instructions for paper mache). When totally dry, paint your helm black. This is the basis of getting a metallic look. Make sure no newspaper print shows through. Then, get some metallic paint on your paintbrush and - here's the trick - wipe almost all of it off again onto some scrap paper. Very very lightly scribble the paintbrush over the helm so that the texture of the paper and decorations is highlighted. This is called 'drybrushing' - if you've ever painted a minature, you might have used this technique. It looks far far better than painting something with solid metallic paint, and much more realistic. Let it dry, then varnish. Do a few coats of varnish. These helms aren't great in the rain and the more varnish, the stronger and more water resistant your helm will be.