The following method of a hardening leather is not my invention. The basic method has been in continuous use for several thousand years. I have studied the subject extensively and the following method is based on what bonafide experts say. I have heard of some people dipping the leather in acetone instead of wetting it with water, but I don't like using that stuff unless absolutely necessary.
The process was known as cuir boulli (boiled leather) in the Middle Ages, but they NEVER actually boiled the leather. (I've tried it - it will totally ruin your leather - TOO HOT - the leather will shrivel up into a tight ball). The process is still used commercially to make cigar cases and such - it was never a "lost" art.
1) "Case" your leather to moldability. The outside aka smooth side of the leather must have no finish on it at this point (if you're doing a pouch type sheath it's OK to finish the inside first and then sew it). To case my leather I thoroughly dampen the sheath or whatever, put it in a plastic bag, and leave it over night in the refrigerator. You want it damp not soaking. It is properly "cased" or "sammed" when the color is almost the same color as a dry piece, but it will feel "cool" to the touch and act like semi-stiff putty. This is the same state you bring it to when tooling it.
2) Mold and bone it to shape if this is desired, but don't let it get too dried out. You can re-moisten it by wrapping it in lightly a dampened towel and putting it back in the plastic bag for a while.
3) Here is the process that makes leather hard - semi-rapid dehydration from the damp state. Dry your item between 120-160 deg F. I use a thermostatically controlled food dehydrator for this, but you can also do it by making a 2-3' square wooden box and setting a hot plate in it or set it up with a couple of light sockets with 100 watt bulbs. Use an oven thermometer and make a little sliding door so you can control the heat. A small fan to move the air around inside the box will help. Suspend the leather so the hot air can move around all sides. You need to experiment with temperature and the wetness of your leather until you get the hang of it. The thickness of your leather determines a lot as well - thinner leather dries faster and can wrinkle or shrivel very easily. I'd recommend 5-7oz vegtan for small sheaths and 8/9 oz for larger ones.
KEEP AN EYE ON IT! Too hot and the leather will either shrivel up or turn out so hard that it will crack. Too low and it won't get hard enough (you can always re-wet and re-dry). I've found best success at around 130-140F for good stiffeness. You can also dip it into a pot of 160-180F water and then put it in your drying box. EXPERIMENT WITH SCRAP if you do this one. This will make it dry faster and harder. I don't find it necessary.
4) Leather dye will also stiffen leather to a degree as it dries it out chemically. If you're going to dye do it I suggest it after the above process. I then use first a light coat or two of Lexol Conditioner and then a water resistant top coat of something like Bag Kote or Tan Kote. I have read some fragmentary accounts of how they did use pure melted beeswax or a mixture of beeswax and pitch to waterproof it (Leather drinking Jacks and Bots were lined with brewer's pitch to make them watertight with cold liquids). With larger pieces such as breastplates they sometimes glued in fine woven linen or canvas linings.
I've made leather armour out of 10/11oz and 45/46 oz using this method that will turn a sharp sword stroke when properly padded underneath.
One caveat: this is always a touch and go process - every hide is different even in the same weights.
For even more info -
http://www.personal.utulsa.edu/~marc-carlson/leather/hl.html
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