Everyone finishes their leather differently, so I'm just going to tell you what I would do.
For a plain holster, while it's still damp, give it a coat of oil, and another light coat after it has dried. Buff to a shine using a cotton cloth. When you apply the oil (could use pure neatsfoot oil), I only apply it to the outside. If you do both sides, it just seems to make the leather feel too greasy. Let the leather breath on the inside, whether you make holsters with the grain side or flesh side in.
To get a good finish on your plain holster, beeswax is an essential ingredient. I use huberd's shoe grease because I find it contains enough beeswax to give me just enough protection without leaving my leather goods feeling sticky, while conditioning at the same time. If you want extra protection, apply a wax after you oil it. Wax the edges too. I make my own wax with beeswax, carnauba wax, and olive oil. For the most protection, some people will melt beeswax and then apply it to the edges.
For more in depth information (consider this the tutorial ypu've been seeking) http://www.jarnaginco.com/catframe.html Go to footwear, bootees, and brogans, using the left hand menu, then find the leather preservation link on that page.