Materials and tools you need:
Thin cardboard such as cereal boxes
Elmer's Wood Glue (or similar brand)
Paper Mache Pulp
Sheets of Craft Foam
A rotary tool with a cutting wheel and drill bit attatchments
Spray Paints and Sealers
Step 1: Creating the "base" of the helmet
Next use the strips to run from the middle of the forehead piece to the back. This creates the curvature of the top of your helmet.
Next we will make another circle, this time fitting around the bottom of your head around the jaw, again leaving enough room to wear comfortably. After this is is done, use more strips to join the top half to the bottom circle.
Once this basic "skeleton" is created all that is left is to fill the remaining spaces with cardboard. I again used strips around the same size and made sure to bend them to the curvature of the basic helmet shape. At this stage it is important to continually try on the helmet to make sure everything fits correctly, it's much easier to fix at this stage than later.
Step 2: Paper Mache
Next, cover the whole cardboard structure with paper mache. I used a simple recipe of Elmer's wood glue thinned with just enough water to make it easily spreadable. For the paper I used newsprint.
Around 3 or 4 layers should suffice, allowing it to fully dry between. Make sure not to trap any air between the layers.