It will be easier if you warm up the holster a little before applying any of these procedures. Lay the holster in the sun or near a heat vent for a short time. Triple K's Production Manager recommends holding the holster in one hand and inserting the unloaded gun into the holster with the other hand. With one hand on the grip and one on the holster, twist the gun around to loosen up the holster and break it in for your gun. Additionally, (if possible to do so) I have found that if you can just wear the holster and unloaded gun around the house for a few evenings, the normal twisting of your body as you sit, stand, etc. will help break in the holster for your gun.
If you want to speed up the process, or want a really slick fit, then Triple K Manufacturing also suggests this method. Wrap your unloaded gun in a soft plastic bag that will form fit to your gun, like a clean bread or trash bag. (The bag is simply to keep your gun and grips clean.) Then cover the bag (the part that fits in the holster) with your favorite gun lubricant (anything from saddle soap to baseball glove softener to Neat's Foot Oil, etc.). You can also lubricate the inside of the holster. Then shove the gun in the holster and leave it for 2 or 3 days. When the time is up, remove your gun and wipe any excess lubricant out of the inside of your holster with a clean towel or rag. The holster should have absorbed the lubricant and relaxed to fit your gun. This is what would have happened naturally over time.
Also, Black leather holsters take longer to break in than the Walnut Oil holsters due to differences in the finishing process.