Assuming your holster has been well sealed and finished you will probably have to remove the finish in order to allow the leather to absorb moisture evenly. This can be done using a deglazing compound, usually available at Tandy's leather stores. After the finish has been removed the holster can be immersed in room temperature water for a few seconds at a time, until it becomes just pliable to the touch. Be careful not to allow too much water to be absorbed, which will make the holster limp as a noodle and also result in some expansion due to swelling (which could be a problem later on).
With the holster just barely pliable to the touch, insert the intended handgun into the holster pocket, forcing as needed to the desired position. Then you can use a blunt tool (such as a smooth file handle or toothbrush handle) to form the leather to the basic outlines of the handgun. Don't forget to form the holster's belt loop for the intended belt (using the belt or a facsimile to hold the opening to shape as the leather dries).
As the leather dries over the next couple of hours you can again use the toothbrush handle, or a Sharpie pen body, to bone-in the details of the intended handgun. This work needs to be done a bit at a time, one or two strokes as the leather dries enough to hold the impressions of your boning tool.
Complete drying will take at least 24 hours in a dry warm environment, more in a humid cool environment.
After the piece has completely dried you can renew the color using leather dyes or stains (available in a hundred shades at Tandy's), then allow that to set up for 24 hours or more.
Then you can burnish the edges, which will have swelled and become raggedy looking due to swelling from water immersion. I recommend treating with wax (beeswax, paraffin, candle wax, etc) then polishing with a hard felt polishing wheel turning in a drill press or Dremel tool.
Finally, you are ready to seal and finish the holster to protect it in use. I recommend a good acrylic sealant, which you can purchase at Tandy's as Resolene or Leather Sheen, applied liberally with a sponge, rag, or sheepskin, then allowed to dry for 12 to 24 hours. Then you can apply a final finish. I like Fiebings Bag Kote for small jobs, again available at Tandy's. Applied lightly, two or three coats about 4 hours apart, should provide a nice soft luster and good protection.
When you are done you can add up all the receipts for dyes, sealants, finishes, and other supplies, and you will see that you could have purchased another holster for about the same money and saved yourself a 3 or 4 day project.
Best regards.
