I do pretty much what WC does. Peter Main, one of the best of the best, and others I know, wet the leather on both sides , set it on their carving slab or something else that's waterproof, and then they set a piece of plate glass on top. The ide is the excess water will ooze out the sides and you can watch the color change through the glass.
There are also carving aids like Bee CArve but I find a light cat of Lexol Conditioner works as well -wet and apply the Lexol, but only to the carving surface.