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Put your gun in a ziplock bag, then put into the holster for a while. You might need a gallon bag depending on the size of the gun. Can also use trash bags or layers of plastic wrap
Basically you are adding a very small amount of OD to the gun to stretch the leather. Start small though, leather doesn't like to shrink from my experience

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Do you have any ball glove conditioning oil? Treat the inside with it.

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I'm going to disagree with this suggestion, baseball glove conditioners are designed to make and keep leather soft and supple, which isn't what you necessarily want with a holster.  Holsters are generally formed to the shape of the gun, things like Glovolium or other lanolin-based products will oversoften it and cause it to lose it's shaping - there needs to be some rigidity to the holster.  If you carry and wear the holster long enough, it will get to this state and it's time to replace the holster.  No need to accelerate that process.

Here is our suggestions to Breaking in the Holster

Place the unloaded firearm in a plastic freezer bag, or cover it in 2-3 layers of plastic kitchen wrap. Do not cover the grip.
Insert the bagged/wrapped firearm slowly into the holster, gently twisting it side-to-side to minimize tearing of the bag/wrap.
Once the firearm is completely seated in the holster, twist it about 1/16" in both directions 6-12 times.
Allow the bagged/wrapped firearm to sit in the holster for about 15 minutes.
Remove the firearm from the holster and remove the bag/wrap from the firearm.
Insert the unloaded firearm into the holster, which should now be snug but not loose. If it is still too tight, repeat the above steps until the holster is broken in.