For this example I'm going to construct a Boba Fett helmet from the infamous Star Wars character but these techniques can be used to make almost any helmet. I was able to download some Fett helmet templates from the Dented Helmet forum. http://www.thedentedhelmet.com
The templates are designed to be printed on letter size paper so just open the files with a program like Adobe Acrobat and print them so they fit on letter size paper. The best way to work with the templates is to cut them out and tape the separate sheets together. I then glued them to the cardboard with some rubber cement and then cut out all the pieces with an X-Acto knife.
The first step is to make the form for the helmet base. This is used to help establish the basic helmet shape and is later removed from the helmet. The form is made from templates pages 4, 8, 11 and 12. The patterns on page 4 and 8 are glued together by cutting a slot in each piece on the center line and then fitting them together. Join the patterns on pages 11 and 12 together and cut them from a single piece of cardboard and then glue that piece to the patterns from pages 4 and 8.
Now make the inner helmet surface. This is done by cutting out the patterns on pages 18, 14,17 and 13, taping them together and cutting them from a single piece of cardboard. This single piece is then wrapped around the helmet base form. Getting the cardboard slightly damp (use a misting spray bottle) will make it easier to bend. Then I epoxy the joint at the back of the helmet.
Next make the helmet outer surface- it is made the same way as the inner surface and is wrapped around and glued to the inner surface. Once the glue is dry cut out the cheekbone sections on the helmet inner surface. Now make the cheekbone sections by gluing the cheekbone template parts together and bending the cardboard and tacking it into place using a glue gun. I only use the glue gun on the inside of the helmet. Then I smear white glue over all the joints on the outside of the helmet.