This is the method I used to make the helmet. Not the best, but definitely easiest/laziest, pretty cheap to make too.
Quick tutorial:
1.
Download Pepakura Viewer (free software)
Pepakura is a program use for making paper crafts. Print them out, fold and glue together to get a 3D object. An 'unfolded' wire frame object.
2.
Get the Pepakura File (Kamen Rider, Ironman, Halo, etc)
It's a '.ppa' file. Best Kamen Rider thread I've seen:
http://www.405th.com/showthread.php/22132-Kamen-Rider-And-Guyver 3.
Print out the file.
This is the testing phase. There are no 'scale' with the program/file, so print it out, fold and glue, then measure how many % you need to enlarge/reduce to fit you. Change the % with the printing option (enlarge/reduce when printing)
4.
Glue it to a bristol board.
I am super lazy so I only print out half of the file (ie just the left side). Cut and glue them to a bristol board to add strength and archive. After drying, cut. You only need one side cause you can always flip the pattern to the the opposite side.
5.
Transfer the foam.
Place the pattern on the foam, and cut with a blade. It's easy to cut the foam.
6.
Fold and glue the foam together.
Using super glue, just glue the foams together following the direction on the pepakura program. Tells you which edge matches which edge.
7.
Complete.
Depending on the detail of the file, you may need to add additional details. The KRW helmet file I got is not really detailed so I worked with the established/cut/glued foam helmet, made paper patterns of the details and transfer them to foam. Glue those as well.
8.
White Glue layer.
Mix white glue with water 1:1. Paint on the foam object with a paint brush. This adds smoothness and toughness. It also allows paint to stick on better. You can use white glue as a filler material too.
9.
Gesso.
Similar to white glue+water, its smooths and strengthens the foam even more. Do a few coats.
10.
Spray paint.
I use Kyrlon spray paint, cheap and durable.
Using the foams it allows for smoothness one cannot achieve with the Pepakura program with out resin/fiber glass and major sanding. You can also use this method for what ever pepakura files you find. I made a Pokemon costume (haunter) for the past Halloween.
I've tried vinyl covering the foam instead of spray paint. It works well, but more time consuming and expensive.