I've read a great many methods of making hardened leather armour, and tried most of them. Here is the method that works best for me. Be sure and read everything, including the disclaimer, before you start.

This process can be dangerous! In addition to the obvious danger of burns, paraffin vapor is explosive. You should not heat wax over an open flame, and should be sure the area is well ventilated.

This is not just a disclaimer to protect me from litigation, THIS IS A WARNING OF A REAL DANGER.

Heating vegetable-tanned leather in the presence of water or wax hardens it, and in the case of wax, protects it from weather. Not being a chemist, I'll make no pretense of explaining HOW that happens, but leather can be made as hard as many modern plastics. If you do it properly, you'll have an item that is hard but not brittle, excellent qualities for medieval armour

Remember, this is a medieval craft so take your time. If you try to work at a modern frenzied pace you'll only ruin your leather, lose your patience, wreck your nerves, pull out large clumps of hair, set your garage on fire, curse me, curse your leather merchant, and in general have an unpleasant experience. Slow down, and tend your work carefully. Every time I have ruined a piece it has been because I left it unattended in the hot wax.

I've had the best luck so far using a roasting pan (I use the disposable aluminum pans that are about 18" x 24") and plain paraffin wax. If you use a pan this thin you should place it on a cookie sheet or two to support it and to diffuse the heat so that there are no "hot spots" in the pan.

I normally soak leather in water and then dry it in the oven on very low heat when I am forming it into compound curves (such as knee and elbow cops). The tighter the curve needed the longer the leather should be soaked, as much as overnight for some items. Allow the leather to drain most of the excess water out before heating. Leather formed this way will retain its shape much better than leather formed after waxing, especially if it is later subjected to heat. Don't get in a hurry and use too much heat or you will ruin your work.

The oven temperature should be low enough that you can hold the oven rack in your hands without being burned, but warm enough that it is uncomfortable to do so. Remember, too little heat just makes the process slower, but too much will bake the leather brittle. I lay several sheets of crumpled aluminum foil on top of the oven rack so that the leather won't get dark places where it touches the rack.

If you are persistant and patient, you can get very tight curves in leather using only your hands. As the pieces dry in the oven, take them out about every 15 minutes and shape them a bit more. The last knee cops I made have no seams but are curved enough cover the top, bottom, and sides of my knees completely (they're for Roman style greaves). I only use a form as a point of comparison, all the forming is done by stretching and forming the leather with my hands as it dries. If the leather dries too much before you have achieved the shape you want, just soak it briefly again and return it to the oven. If you have sewn the leather anywhere, the stitch holes will enlarge a little as it dries. You can remedy this by lightly hammering them occasionally to close the holes back up, if this is important.

You should make sure your leather is COMPLETELY dry before waxing it. Any places that are the least bit damp will end up too brittle, and will shrink and twist a lot.

The wax should be only slightly hotter than is necessary to keep it melted. Too much heat will actually cook or burn the leather, and it will shrivel and become very brittle. When a piece of leather is dropped into wax of the correct temperature range, tiny bubbles will stream up almost immediately.

Here is where my method differs from the others I have seen: I leave the leather in the wax until nearly all the bubbles stop! Watch the piece(s) carefully, as the parts that lay on the bottom of the pan can burn by getting too much heat from contact with the pan bottom. Most heavy vegetable tanned leather takes from 20 to 45 minutes to absorb the wax properly.

Your pieces will be somewhat flexible when first taken out of the wax, so support them in the shape you want as they cool. While you can do some forming at this stage, you should have done all or most of it before waxing. Shaping done at this point will relax if the piece gets hot later. Nothing like opening your car trunk in summer to find that your leg armor is as flat as your heater.

With the wear and tear of full-contact fighting, armour pieces often get bent or dented. Simply put them back in a warm oven, and reshape. This will also cover a lot of scratches, since the softened wax flows to fill small holes and roughened areas.

Leather is, for most of us, an expensive raw material. Time spent experimenting on scrap pieces will be well invested, and may save you ruining a large piece later.