So! I figured I would chronicle the steps I use when I make cosplay armor. I find that craft foam works really well! This is the same technique I used for my Deedlit, Umi Ryuuzaki and Lina Inverse armors in my cosplay gallery.
Here are the materials you will need:
Tape (either packing or masking, doesn't matter)
Ruler at least 12 inches long
Pen or pencil
Craft foam (several sheets; at least 3mm thick. I used 6mm here)
2 liter bottles if your armor is curved
Fabric in the base color of your armor (I highly recommend a
basic cotton mix, like broadcloth)
Thread in the color of your fabric
Either a hand needle or a sewing machine
Let's begin! I'm demonstrating on my Black Sun armor for Princess Azula (Avatar: the Last Airbender).
1) Grid out a full piece of posterboard in inches or centimetres, whichever you're most comfortable using.
2) Trace out a basic pattern for the piece of armor you'll be making. The gridlines are to help with measurements and proportions. You'll see in the picture above my pattern piece to the left. If you find that you don't have enough posterboard in one dimension, tape a couple of pieces together and re-grid. Really large pieces might take quite a bit of taping!
3) Hold up your pattern to yourself in the position the final piece will be in (i.e. if it's shoulder armor, curve it and hold it on your shoulder). Check the fit and proportions. Adjust as needed--you can always tape more posterboard together.
4) If you're satisfied, take your pattern piece and trace it onto your craft foam. Again, if you're short on foam, use Tacky Glue to glue the edges of two pieces together (make sure it's the same thickness foam), wait for it to dry, then trace. Cut out the piece of foam.
5) If you need to shape the foam to a curve, I found that the least time consuming way was to wrap the foam around a 2 liter bottle, wrap a strip of newspaper around the center of the foam, and tape the newspaper strip down. (Don't tape directly to foam--tape will destroy the foam when removed.) Let it sit for about a week while you're working on the rest of the costume. When you remove the tape/newspaper strip, the foam will hold its curve. You can see two of my shoulder pauldrons sitting on 2 liter bottles in the upper right corner of the picture above! (You can also shape foam over a low heat stove burner or with a hair dryer if you don't want to wait a week.)
6) Trace your pattern PLUS 5/8" ALL AROUND onto your preferably-preshrunk fabric; then, flip the pattern piece over and trace it again (make sure you include the extra 5/8" all around!) for a total of two pieces. The extra 5/8" is your seam allowance. The two pieces of fabric will be sewn into a sandwich, or sheath, for your foam core. My fabric sheath is on the right in the picture; I didn't finish pinning it.