Mix together four cups of water, two cups of flour and a quarter cup of white glue. Tear non-glossy scrap paper into 1-inch-wide strips. Put a handful of paper in the glue mixture to soak.
Blow up a large round balloon to a size similar to that of the intended wearer's head. Pick one of the strips of paper out of the bowl and smooth it onto the balloon. Continue until half of the balloon is covered. The paper covering should resemble a cap. Let the layer of paper dry. Add three more layers, letting each layer dry between applications.
Cut a piece of tag board the right size to cover the back of the person's head from just behind one ear to just behind the other, and from the crown of the head to the neck. Cut two pieces that are the right size to cover from just in front of the ears to the nostrils on the wearer's nose, and from the hair line to about two inches below the chin. Cut a piece of stiff plastic film the same length as the two cardboard pieces, and wide enough to cover from temple to temple on the wearer.
Lay the film down on a table. Place the two smaller pieces of tag board on top of it. Mark two eye slits in the cardboard, and cut them out. Draw an arc from about 2 inches from the center opening to about 1 inch below the eye slot. Cut along the line. You should now have a two pieces of tag board with an arced cut from the bottom front to the outside edge under the eye slot.
Cut two 1-inch strips of tag board. Mark the centers of each end. Cut the ends so that they taper off to a point. Use paper tabs to glue the tag board strips to the arced cuts. Glue the pieces you cut off the front pieces to the other edge of the taped strip. Brace the structure by taping the back edge to the plastic film, using masking tape.
Pop the balloon armature that is under the dried papier mache. Glue the back piece of tag board to the back edge of the papier mache cap. Leave a space for ears, and glue the front assembly to the other side of the cap. Add reinforcing strips of tag board around the top of the helmet, and along the edges of the back skirt and around the face plate. Paint with tempera paint. Use a sea sponge to add blotches of slightly darker color or brown and orange to simulate rust. Let this coat of paint dry.
Paint odd blotches and paint chips using a small brush dipped in black tempera. Cover the plastic film with some scraps of tag board, and spray the whole thing with spray fixative or other clear finish. If the clear finish is brushed on, be careful not to smear the details.