I made this for protection during cardboard tube tournaments ( http://www.tubeduel.com/ ). It protected me well; I took second place and I believe I would have won had the final match been allowed to progress to its natural end. Damn refs, grumble grumble.

I used the following as a general reference, and for the names of the parts (several of which I did not previously know):

http://www.reliks.com/suitofarmor.ihtml
http://www.swordsandarmor.com/mall/suit-of-armor-Dragon-Slayer.htm
http://en.wikipedia.org/wiki/Plate_armour
http://en.wikipedia.org/wiki/Cuirass

Step 1: Breastplate and Backplate

Having quite a bit of experience making clothing, I figured fitting cardboard around the body would not be too difficult. As it turns out, there are some differences between the stiff cardboard and the much more malleable fabric. I worked out a technique of scoring the back of the cardboard with the cap of a pen along lines I wanted it to bend on. This, along with a way of making darts, allowed me to shape the cardboard fairly accurately.

For the front and back plates (together these are called a cuirass), I first drew a flat shaped based on my body measurements. For the front I used these:

front of the waist (a little more than half my waist measurement)
length from waist to shoulder across bust
length from waist to bottom of neck hole
front of the bust (quite a bit more than half my bust measurement)
width from arm to arm, above the bust
length from waistline to underarm hole

For the back I used these:

back of the waist
length from waist to shoulder
length from waist to bottom of neck hole
width from underarm to underarm
width from arm to arm across the shoulder blades

I drew the front with darts at the bust so the armor plate would fold out there. (I didn't bother to put any darts in the back plate and this was fine.) A less busty person would need less of a dart, but most people will need at least a small one. We're all just curvier in front. I did place the waistline for each piece on the edge of an existing fold, where the original box had an edge. I also left about an inch of extra cardboard there out of habit, but it turned out this was a good thing (see Step 6 about the pauldrons).

Cut out the pieces with a knife of some kind - a utility knife or box cutter works well. Cut only the lower side of the dart marking all the way through. The upper side, cut through only the top layer of cardboard. Then peel the top layer and the corrugations off, revealing the inside of the corrugated sheet. It looks like a small flap. To construct the darts, bend the cardboard so the dart edges meet, and hot glue the flap down to the outside of the front plate.

It's easier to bend the cardboard if you score the inside first, just run the cap of a pen firmly along the line you want to bend on. A stiff ruler helps a lot for scores that run diagonally to the corrugations. In addition to the bust area, score and bend the shoulders and sides.

Step 2: Helmet Part 1

It's possible to really do a cardboard helmet well, but I chose to go a faster and easier route. I measured around my head at the largest point (about the forehead level) and cut a strip of cardboard that long plus one inch for ease and one inch for overlap. The height I estimated by holding a ruler up to my head in a mirror.

I cut the helmet piece at the edge of an existing edge of the original box, and went over about an inch. I made a lot of vertical scores, after which I was able to easily roll up the cardboard. I sliced small V's out of the part past the existing foldline, so they wouldn't interfere when the cardboard was curved into a circle.

I drew an oval with a circumference of the same length as the head measurement, which in my case was 24". Most heads are oval shaped to some degree, you nearly never get a very circular one.

I wrapped the bent strip around the oval and marked where the overlap came to, and hot glued it together into a tube. I pushed the crown to the top where the V-shaped flaps were, and glued one flap at each end to keep it in place. Then I glued the rest of the flaps, one at a time. I found it was easier to put the glue on the flaps than on the crown piece.

Once that was done I decided to reinforce the crown with a second piece of cardboard on the top. I simply traced the outline, cut another piece, and glued that on.

Step 3: Helmet Part 2

Now you have a full head helmet but no way to see out! Not very useful. Work out where the face opening should be and cut the helmet open there. Start small and work your way out.

Step 4: Vambraces

The vambraces guard the forearms. I didn't want to attempt cardboard gauntlets... maybe someday.

I measured my wrist and my forearm for circumference and length. Then I drew a shape like a thin conic section to fit the measurements. On the CMS cardboard, though, I forgot to allow space to glue the vambraces together so I had to start over.

Note the diagonal scoring I did, again with the cap of a pen and the metal ruler. This allowed me to easily bend the cardboard into a conic section that would fit my arm, instead of a straight tube that would not.

If you have large hands compared to your wrists you may want to make the wrist area larger and maybe tape the vambraces to a pair of gloves to keep them on. In fact having worn these, I strongly recommend gloves! My writes got rather chafed on the edge of the cardboard.

Step 5: Pauldrons

Pauldrons are the bits that protect the shoulder. I haven't ever seen a suit of armor with its arms up, but it seems logical that you'd want to keep that range of motion, if possible. So I decided to make pauldrons rather flatter than in my reference picture, and I would simply tape them in place at the shoulder edge and let them hinge up when I raised my arms.

This was the most complex shape I'd yet attempted. I made a paper model first to test out the fit, and found that a pair of mostly symmetric pointy oval shapes worked well if I put a small dart in the one in back. Really just look at the pictures for how these work. Again I used the technique of V-shaped cuts to attach one piece to the other. And I did the dart the same way as the breastplate darts, peeling away half the cardboard layers.

To attach the fronts and the backs, a fair amount of force was needed to keep them shaped properly for gluing. I glued the flaps one at a time and let the glue cool completely before going to the next. This took some time but I was rewarded with a really solid construct -- they took a lot of hits and came out of the battle looking as fresh as when they were made!

Step 6: Faulds

I'm not sure these are really faulds or if they're just part of the cuirass... but whatever. The bits that stick out from the breastplate & backplate to protect the hips.

Here again I had to attach a curved piece to a straight piece. The shape of the faulds I worked out on paper first, remembering that in order for them to stick out properly, the whole thing had to have a slight curve. After cutting out the pieces, I also cut the familar V-shaped slices in the bottom strip of the front and back plates to make gluing flaps.

Glue one flap at a time, starting in the center. You may have to put a fair amount of force into holding the breastplate or backplate in place as you glue but curved shapes are strong and once the glue is cool the construction will be quite strong.

Step 7: Wear With Pride! But First You Have to Get Into It.

A trusty squire is vital at this point. Tape the front & back together down one side and see if the shoulders fit together. Bend them more if necessary. Now ease your way into the armor. Tape the shoulders gently - you may have to get in & out a couple of times before the armor fits properly. Try to bring your arms together in front. I had to cut away about an inch on each side of the armholes before I could do this comfortably enough to hold a cardboard tube sword.

Once you fit properly into the cuirass, or the cuirass fits properly onto you, duct tape both sides and shoulders closed. If you can, tape a bit to the inside as well.

Next have the squire affix the pauldrons to the shoulders, about in the middle of the shoulder strap. Remember that the dart goes in the back, and nearest to the neck. Use duct tape on the top and bottom, and reinforce with strips of tape over the edges of the hinge tape pieces (er, just slap a lot of tape on there).

I also had to cut shoulder-shaped curves in the edge of the helmet as the cuirass stuck up higher than my regular shoulders. And a bandanna or balaclava or headsock is useful for Hair and also gives that good medieval look.

(Sorry for the blurry pix here, we were late for the tournament and rushing.)

Step 8: Attack!

Go thou forth and smite thine enemies! Which shall not in turn smite thee, but fall before thine anger like sheaves of wheat beneath the thresher's flail, or be crushed beneath thy feet as a pismire.