Howdy, Pards,
Absolutely NEVER use neatsfoot oil on leather! The stuff is made to soften the hooves of LIVE horses and cattle. It will destroy the fibers of tanned leather! Oh, not immediately... but it won't be long.
The way to tool, dye and treat topgrain vegetable-tanned leather is to wet with water, then tool. Allow the leather to dry flat. Now, as to which dye you use, that's up to you. I personally use alcohol-based Fiebing's dyes. With black dyes, I get best results using a 50/50 mix of Fiebing's Black and Tan. Depending on how many applications you use, you can get anything from a faded black to black-black. (I market this as 'Faded Arsenal' in my catalog.)
Allow the leather to dry thoroughly. Then treat the leather on both sides with Lexol Leather Conditioner(R). But do it LIGHTLY! You can always apply some more, but it usually isn't necessary. And too much can really ruin the leather! Do your stitching and then (in the instance of a holster) you can wet fit the holster to the gun. DON'T soak the leather in a bucket, or anything of the sort. Run it under cool tap water until the leather is moist (30 seconds is usually enough), then form to the gun. (I usually stick the gun in a plastic baggie, wrapped tight around the gun. When wet-fitting a Colt's style gun, with the ejector on the lower side of the barrel, make a "tunnel" from the ejector thumbpiece toward the top of the holster, rather than letting a pocket form, which can trap the gun. Allow the holster to dry for a couple of hours with the gun in it, then remove the gun and allow the leather to dry overnight. Try the gun in it, and form the holster a bit more until it is the looseness desired. You may need to use a 7/8" dia. hardwood dowell with the end rounded slightly to do the initial forming of the holster "pipe" for the barrel. Use smaller hardwood dowells for forming the trigger guard and the extractor thumbpiece while the leather is still wet.
Hope this is of some help.
Remember 9/11!