This tutorial will be one of several I plan to post on making foam armor. This will focus specifically on the helm, and the rest will come as I am able to complete those projects.
Let's begin with some basic information about the materials and processes we're going to use in this tutorial. The primary material I used was EVA foam (Ethylene Vinyl Acetate). This is a polyethylene and vinyl acetate copolymer that is commonly found as anti-fatigue floor padding in hardware stores, and also craft foam in craft stores. Great things about this material are that it is relatively easy to mold and shape, and it's pretty forgiving if you make mistakes. You can use an oven or a heat gun to efficiently mold the stuff into whatever shapes you want. Cutting it requires a very sharp blade, like a new hobby knife, box cutter or, better yet, a hot knife. It's easy to leave cutting marks on the edges of the foam, but those can be either incorporated into the design or even sanded off with a Dremel tool.
To make something like this, you need a simple mold that will give you all of the curvatures you need. Throughout the project, I used a large, thick cardboard tube that was close to the major diameter of my head. Either way, the cardboard tube served as a mold for just about all of the thick foam components I thermoformed, and I would highly recommend it for getting geometrically attractive results. This technique produced a rather circular helm, but the helm can be squished into more of an oval-shape later on if you want. In the following step, I will cover all of the materials you will need to build a helm similar to mine.