Summarize:

Making inserts in your holster will help stiffen it. This helps hold your weapon safely and securely. You're going to need a right, left, and top insert. Simply trace the sides and top of your weapon with a marker onto the inside of your leather. Rough cut it to be the size of your weapon. Cut it with your razor knife.  Place a piece of spare leather between the leather that you're cutting and the table. This protects your table and your razor blade. Cut it bigger rather than smaller. Cut along the outside of the line that you've traced. This way you can match them up and cut them down until they are perfect. It is possible to cut them down more and more, but if you start off by cutting too much off, you can't add to it. After a few hours, go get your holster. It will still be damp, but when you take off the clamps, it should be starting to hold the shape of your mold. Dampen the outer piece of the holster once again. You want it about ¾ as damp as the original time that you dampened it. This means it will be wet, but won't leave water on your hand when you touch it. Use the empty weapon instead of your mold this time. It will give the leather a better fit around it. Use your stretching blocks and re-clamp them over your holster with the mold and inserts inside. Place your holster back in the sun to dry. The longer you let it sit and dry, the better shape it will hold on its own. If you don't let the leather set in the proper position, your weapon won't fit correctly. This could cause it to fall out of the holster, or it could create difficulty for you when trying to get it out.
Stiffen your holster. Cut your inserts. Get your leather holster from outside. Place your inserts on the inside of the leather, around your weapon. Let it sit for 24-48 hours.