Tbird,
Since I built a flat backed holster, I only molded the front piece. I used 8 oz veg tanned leather. The leather itself must have been real high quality, because it molded very well. I basically ran it under hot water until it was soaked, let it dry until it became cool to the touch, then began to mold. I stuck the pistol on a towel on a counter and put the leather over the top of it. I used several different things to mold...the rounded end of a ratchet, screwdriver, etc. I just started with the higher parts such as the grooves in the cylinder, the space between the cylinder and barrel, and the grooves in the barrel. After they were good and molded, I moved down to the other parts such as the trigger guard and the outline of the pistol. I let it dry on the counter overnight with the pistol still in the molded piece with two heavy weights on the leather where the clips attach to keep them from curling upward. Then I sewed the two pieces of leather, slightly wet down the molded piece and did some more finishing molding with the pistol in the holster. I let it dry again with the pistol in the holster.
Nothing fancy I guess, but I did get the leather from a custom leather worker here in town and I really think the quality of the leather had a lot to do with how well it molded.