Leather is a popular material for clothing, shoes and other garments because of its durability and flexibility. It is created by tanning animal hides, generally from the skin of cows. Leather can be made so durable that medieval knights used to use it for armour. The cuir bouilli technique was used to make not only leather armour, but a variety of other useful rigid objects such as cups and bags. Whether you would like to shape your leather into an object or just stiffen it a little, you can use this simple technique.
Heat a pot of water to 180 degrees.
Place your piece of leather in the pot for approximately 60 seconds.
Watch for the leather to darken, go limp and curl up. Once it does this you will know it has hardened. If you would like to stretch and shape the leather or just want it moderately stiffened, remove it at this point. You will have one or two minutes after removing to stretch it before it loses its stretchiness, though it will still remain flexible.
Leave the leather in the water for longer if you would like it harder. It will continue to get harder, darker and shrink until it resembles wood. The trade-off of such hard leather is its brittleness.
Let the leather air dry on a rack. It will continue to stiffen.
Leather is an organic substance, meaning every piece is unique and will react differently to stiffening. Do not assume additional pieces will stiffen in the same amount of time or react as well to the same temperature. It is best to stiffen samples from each piece of leather that you intend to use.