The leather is wet until it is damp, then "cased" or allowed to dry while wrapped in a towel or something until it is about the consistency of firm putty.
At that point it can be wet-molded to the exact shape of the gun using smooth tools of the proper shape to press the leather into the contours of the gun..
When it dries, it fits the gun perfectly, and after finishing with some sort of leather dressing oil & finish will retain it's shape indefinately.
rc