If you go to the "hidden leather armor" thread in MComb, you'll find it discussed.
I've had pretty good luck stretching the leather by nailing it onto a last (2x4 works fine for small pieces), and then soaking it until it's wet. Let sit a bit, start working wet hide glue into it one thin layer at a time, using a glue-gun to keep it thin enough that it penetrates.
Kilkenny's not really been satisfied with those results, and uses a water-glue mixture. I've done that, as well, though not as much. Details in the other thread.