For initial forming, I use water to soak the holster during boning the shape of the pistol to the holster, and the nice thing about doing your own, you can put in on your body, and let the holster take the shape of your body contour. If the holster is already made, I soak in denatured alcohol, pistol wrapped in saran wrap or a bag, then put it on the body, the advantage is it dries almost immediately, sometimes will remove some color, and it will need to be oiled again to protect the leather.
Warning: Alcohol vapors are highly flammable, do not use the technique near any open flame. The vapors can be very strong to the nose and eyes, so only do in an open, airy area.