Leather can be stiffened in many different ways, but the main way that leather becomes stiffened is applying some type of liquid on the leather and allowing it to take a different shape through the use of heat. There is no correct or incorrect way to stiffen leather.
The most common method for stiffening leather is to allow the leather to soak in cold water. It can be soaked anywhere from 15 minutes to 12 hours. It can then be formed into the desired shape and left to dry until it is completely hard. Another method for stiffening leather is hot water soaking. The water should not be too hot; it shouldn't be uncomfortable to put the hands into. The leather can be soaked until the water becomes cool and then should be left to dry. Leather will generally be more stiffer and more brittle if it is placed in hot water. Leather will shrink and become extremely hard when placed in boiling water. To speed the drying process of stiffening leather, it can be baked in an oven. The oven should be no more than 122 degrees Fahrenheit and the leather should be watched carefully to ensure that it doesn't burn or shrink too much.