I’ve done a lot of this on brain tanned , and chrome tanned leather. First off if it has any of the yellow dye used years ago you have to get rid of it. That pretty easy actually. It’ll scare the bejabbers out of you but it works. You simply put it in the washing machine with some Rid dye remover, with warm water (NOT HOT), and continue resetting the cycle to agitate so it doesn’t pump you dye remover down the drain, until the yellow is gone. Hang it in a cool dry place, out of direct sun light, until it is damp dry. Now throw it in the dryer with a couple of tennis shoes, and run it on air only ( NO HEAT). Now to smoke the leather you can use whatever you have, be it a tent stove, the top vent of your propane smoker, or just a teepee shape frame made of sticks, over a fire pit in the yard. Different woods make different colors some good, some not so much. You want wood that is in an advanced stage of deterioration, but not wet, and rotting. I personally prefer cedar for color, and smell. You need to keep the fire very low, so the heat doesn’t damage the leather. Then just smoke it until it’s the color you want, rotating it regularly so it comes out uniform in color.
Hungry Horse