There are many different methods to harden leather. The main methods are laquer, wax, and water.
Laquer is rare, mostly seen on samurai style armor.
Wax is popular because its easy and waterproof, but adds weight.
Water hardening is a good place to start, I water harden all armor before waxing. It shrinks in size, gets thicker, denser and wants to keep the shape it dries in.
Water hardening is often called "Cuir boille". Despite the name don't boil your leather. I think that cuir boille refers to the bubbles that the leather makes, not the water temperature. If you use boiling water for longer than 30 seconds it will be brittle and useless. The trick is to keep it just under the point where it gets brittle.
I've had good luck with 76C° for 80 seconds, but there are several methods of water hardening. Oven drying at low temp is another method. The greaves in the pic were held under warm tap water, shaped and dried hard in the oven at 40C°. I've read of traditional methods that soak the leather in diluted glue then dried in the oven. I've heard also of boiling water for 20 seconds then oven, but that seems crazy to me.
Ok, so you want to water harden leather for the first time.
First test your leather. Seriously there's a lot of variation in how much it shrinks. I cut an exactly 10 cm strip, cook it and when it's mostly dry measure it carefully. Mine typically shrink about 10%. This is important to add to your patterns before you cut. Honestly when making a new pattern I usually underestimate how much the leather shrinks and end up with a mini demo before I get the pattern to work.
Next water shape. I run a warm water from the tap, maybe 15 seconds per side. give it a couple minutes to "dry" as the water soaks into the center of your leather. Now you can shape it. I roll, stretch, and dish it depending what I need.
To dish, I pull it over a bottle top to get a small cup, then push it into a bowl being careful to avoid wrinkles. Once you have it in the shape you want, dry it in an oven. Mine on low with the door held open with a spoon is about 30-40C°. after a few hours you should have some medium hard leather which is closer to flexible than brittle. This should pass for boffer armor.
Then I water harden. I put it in 76C° for 80 seconds. This is when it will shrink and get thicker. There is a magical couple of minutes when you take out the leather. For a short while it will be stretchy and you can put some final shaping, dry it again, and hope it didn't get too hard and brittle. Now it should be quite firm, but still have a little flex. Here is the first video, by Art of Warriors i used while learning.
Finally use a bit of sandpaper to smooth out the rough insides and edges. Assuming I'm in to much of a hurry to bevel and slick my edges. I generally dye after water treatment. Then it should be sealed waterproof with resoline, or supersheen. If you intend to wax harden, don't seal it.
Next I wax harden. I can think of at least a dozen different methods to wax harden, with different blends of waxes. We'll save that for the next post.
I am not some leather guru.
I'm an enthusiast who is sharing my experiences. Please don't think this is the "right" way to harden leather. There are many different methods to make hardened leather armor. This is my method today. Tomorrow I might do something different. I'll let you know if it works.