Forgot to cover the treating...sorry.
Treating the leather:
If the leather is "green" (untreated) vegetable tanned leather, then I like to wet form the quiver before adding any oils.
Wet forming is just what it sounds like. Wet the quiver, not a dripping soak, or a light rub of a damp cloth, but wet it so it's pliable. Wear it when pliable to get the shape you want and use your hands if necessary to form it to your needs. Be careful not to crease the sides where the body begins to collapse, but rather allow the body to collapse with a nice rolled over transition from front to back.
Let dry completely.
When dry, treat it with your favorite leather treatment
Mink Oil paste
Montana Pitch Blend
Neetsfoot oil
Fiebing's Aussie Conditioner with bee's wax (what I use mostly)
Pure Olive Oil
Basically anything that does not have petroleum distillates in it, such as Neetsfoot Compound.
Treat it and flex it. Let the treatment soak in.
Repeat/treat/flex until the desired look/feel is reached. The more treatment you put on it, the darker the leather will get. You can do too much. If you do too much, the treatment will trasfer to your clothing easily.
You can also treat after cutting out your leather parts but prior to punching any holes on your leather. Then after you get the desired soak of treatment, punch the holes and assemble. Treating with punched holes can allow the treatment to soak into the holes more so than the leather surface of the body. It can look splotchy.
I use a cheap paint brush to apply the oil/treatment. Sometimes I use my bare hands.
This is where my wife gets upset occasionally. The kitchen table gets oiled alot. Use old towels as a work surface, if possible, on top of your workbench, countertop, table. Keep the wife happy.