You don't want to stretch it much. Do not wet the holster. Take a heavy plastic bag, the one the holster came in works good, and put your gun in it and put it in the holster overnight and see how it is in the morning.
If it isn't stretching enough, wrap the gun in saran wrap before putting in the bag.
The key here is patience, take your time stretchng so that it doesn't get stretched too far, causing a fit problem that can't be corrected very easily.
That is how I have stretched all the holsters that I have had to stretch.
bob