Hardening leather armour makes it less susceptible to blunt weapons. In medieval Europe, such armour was called cuir boilli. That refers to the hardening process, in which the leather is boiled until it has a hard, wood-like density. Whether medieval warriors used wax to treat their armour is a subject of debate, since wax-treated armour is more susceptible to edged weapons than soft untreated leather. The water-boiling process is a means of hardening leather that doesn't require the use of expensive waxes.
Determine your project needs. If you desire leather that's only somewhat hardened but still flexible, you'll be using a shorter immersion time, and your armour can be moulded to a shape you desire. But you can also make extremely hard slats of leather as well, depending on the style of armour you desire. Your leather will shrink as you harden it, and the amount of leather you will need will depend on your project dimensions and armour density.
Soak a piece of vegetable-tanned leather in water for about ten minutes. Heat a pot of water to 82.2 degrees Celsius and immerse the leather in the water and watch it carefully. The longer the immersion, the more the leather shrinks, and the thicker and harder the piece grows. After an extremely long immersion, the leather will have the consistency of wood, which may suit your purposes more than a moulded article. Experiment with different periods of time, and have an idea of what the leather looks like at different stages of the process to create pieces with a uniform density.
Cut the armour into scales, if you're making scale-mail leather armour, and then add holes with a leather punch while the armour is still flexible. The scales can be any shape or size that you like; look up different armour styles online to find the look you're considering. Most hardened leather armour is based on fantasy designs; but for a traditional look, make your scales in disk shapes, about ten inches wide and eight inches tall. You can then rivet them onto a cloth or soft leather shirt.
Wrap a towel around your arm to protect it from heat and to account for padding, and tie a piece of recently-boiled leather around it, holding it in place with strips of cloth. After about fifteen minutes, release the leather piece and keep its shape -- it should still be flexible -- and let it sit. Cut off any excess, and you'll have a simple bracer.
Construct other armour pieces based on the process of heating, moulding the leather using a pattern and riveting or sewing it into a suitable shape. If you make a mistake, and the leather hardens for too long, and it's too small for your form, for example, you can simply flatten it out and cut the leather into scales for later use.
Cut pieces slightly larger than the design calls for to account for shrinkage