This is some armor I made from craft foam from Hobby Lobby(79 cents a sheet). After a little planning and a lot of effort, this is the result.

Step 1: Materials

Here's what you'll need:

Craft foam(once again, Hobby Lobby- 79 cents)
Scissors
Glue gun
A good plan, and probably use a drawing.
Velcro

The picture is the foam, which is 2mm thick, I believe.

Step 2: The Idea

Get a drawing, preferably. If your memory is that good, well, you're lucky.

If you have a turned(3/4) view, you will want to draw the front and back as well.

Unnecessary note: I later scrapped the "gas mask" look for the "made in one night" look.

Step 3: Draw the Design and Cut It Out

As the title says, in this step you should draw the design on the foam and then cut it out. I you don't want to have to glue each individual piece(which I didn't, but whatever), then just push hard when you are drawing on the foam. I ended up cutting out the pieces and then covering them with another sheet, which I then drew the design on again.

Step 4: Glue

Glue the foam pieces down on another big piece for the backing. I messed up on the back a little and made it lopsided, but it wasn't a big problem.

Step 5: Glue Another Piece Down and Add Velcro

Add another layer of foam to the top(if you want to, but be warned: It gets pretty stiff). Although if it gets stiff, you can probably make it less so by using the armor a lot. Add Velcro to secure the sides.

Step 6: Complete the Costume.

Use anything else you want on the armor to make it suitable for the occasion to which you are going. I made a face mask and glued it to a hood, but use whatever looks good to you.