My son said he wanted to be a samurai for Halloween, and that it had to be blue (Thanks, Age of Empires II). This was my first foam project so there was a bit to learn, but it's incredibly easy to work with and forgiving of mistakes. Come see how it's done!

Step 1: Supplies

Materials:

Spray paint: Primer, black, metallic gold, blue, clear coat glossy. Every website I've seen recommends Rustoleum brand.
Blank mask from craft store
Toy army helmet from Dollar Store
Cardboard tube for helmet fukigaeshi (wings)
EVA foam. I used 3 squares of the 2'x2' floor mats sold in a 4-pack at Harbor Freight.
Craft foam. I bought several sheets of the thin EVA foam sold at Craft Warehouse or Michaels, for use in detailing.
Nylon rope. Sold at hardware stores, got mine at Home Depot.
Nylon webbing. Used the 0.5-inch size. Sold at sewing supply stores. I tried some cotton webbing but it would not grab the clips.
Clips for nylon webbing. Bought a 30-pack at Craft Warehouse for $5. Used the 0.6-inch size.
Crazy glue
Self-stick "bumps". Don't know the proper term. These made the studs on the helmet. Came in a sheet of about 200 for $1 at the Dollar Store.
Acrylic paint for aging the armor. Black, burnt or raw sienna.
Bondo for filling cracks.

Tools:

Heat gun. Hairdryers reportedly don't get hot enough.
Dremel for trimming
Hot glue gun & LOTs of glue sticks
X-acto knives and sharpener. You literally need to sharpen your blade once for about every 20cm of foam you cut. It's powerfully dulling.
Disk sander for smoothing edges. Could be done by hand as well. 80-grit for shaping, 120+ grit for smoothing.
(optional) Silhouette Cameo. This is the craft cutter used to cut out the stencils for the helmet designs.

Step 2: Templates

Your child may vary, so the templates here may need some adjustement. They should give you an idea of the shapes you'll need, though. Each small square on the green mat is 1/2 inch. To make these I simply taped paper to my son's torso and traced out cut lines.

I've also included the templates for the helmet crest and stenciled designs on the helmet. These can be opened in the freeware vector editor Inkscape.

The plate armor did not get a template. I think they were made of 2"x3" rectangles. You'll need 20 of these, 5 for each shoulder and hip. The top of the shoulder armor never got a template, but you should be able to figure it out from the picture here.

Step 3: Helmet

The mask of a samurai helmet covers only the lower part of the face, which is nice because it allows for better vision that way. Probably not a coincidence.

I cut off all but the front brim of the helmet with a Dremel.

Using the thin craft foam and glue gun, I covered the army helmet. I realized the neck protector was too vertical, so I cut several slits along the bottom edge and opened it up, then glued foam triangles into the openings to hold it there. I also added horizontal strips of foam to give it a layered look. I Bondo-ed the top to fill cracks. Then glued on thin strips of foam for the vertical banding. Then it was fun with self-stick bumps to make the studs.

The fukigaeshi are just ~3-inch segments of cardboard tube cut down the middle and folded. After a primer and black coat of paint, cut out the stencils and sprayed them on with gold paint. I recommend spraying stencils with craft adhesive spray before setting on the target, to prevent lifting and smearing.

The helmet crest was cut out of 3mm birch ply and coated with gold paint. I stuck it on with a small square of foam and hot glue. The foam allows for a bit of "give", so the crest is less likely to break with minor jarring.

Step 4: Cutting, Molding and Gluing the Armor

Trace the templates on the previous page onto your foam. For items with a left and right version, be sure to flip over the template and trace the other version. The chest armor here was made from one large front piece and two back pieces crazy glued onto the front piece.

Before any assembly I added the ridges at each opening in the armor. On the back side of the foam (the textured side) I cut a line 1/4 inch in from the edge, about 2/3 of the depth of the foam. I then glued a string down in this slit, propping it open into a "V". On the front side of the foam this left the ridge.

As for bending the foam, Youtube is full of how-to videos I learned from, but basically you heat gun both sides of the foam, bend it, and hold in the new shape until it cools. It will spring back a bit when you let go, but you can repeat the process as many times as it takes.

For edge-to-edge gluing, I used Crazy Glue, though I have seen people using hot glue or a variety of "contact cements".

The straps are simply hot glued on, and have held up just fine. Tip learned from painful experience: Make the strap for the male end of your clip far longer than you think you'll need. The blurry photo here of the back side shows the thin foam triangles glued over the strap ends to make it look more "period".

The armor plating is held together by rope. The pattern shown in the photos here works well. Be sure to tie knots before and after every joint-- looks better and prevents slipping. The armor plating is held to the belt and torso by hot glued straps.

Step 5: Painting and Weathering the Armor

Each piece got 2 coats of primer, then 2 coats of the main color, then weathering details, then 1-2 coats of glossy top coat.

The weathering did a fantastic job of taking the armor from looking like a plastic toy to looking like metal or worn leather. And it's so simple, my 7-year-old did most of it.

First I airbrushed black accents into all the armor folds. This step was probably not necessary.

Next, I mixed a thin wash of black and sienna acrylic paint. My daughter painted it thickly into all the foam joints. She then applied a wash over the rest of the armor and blotted it off with a wadded cloth. Finally, I dry brushed on highlights (just primer paint and the main blue paint sprayed into a jar).