Once you are ready to form the holster, wrap the pistol in plastic wrap, it probably wouln't hurt to oil it lightly prior to wrapping. Once it is good and wrapped, wet the holster. Insert the wrapped pistol into the wet holster and form fit it, using an appropriate tool. Then set it aside to slowly dry. Once dry, remove pistol, treat the leather.
I have not done glass blowing in years, I am a chemist by trade and when I was in school, we had a class in glassblowing. We were taught to make many laboratory items. I did a wine glass as my independent project though. Fun stuff, just haven't had a chance to do it in years. I remember the teacher laughing everytime someone would scream from a burn and follow it with "hot glass looks just like cold glass, be careful". Had a sadistic streak.