To darken a stock:
Tap out a little bit of bone black and pour a little bit of finishing oil into separate piles on a non-absorbent surface. First, dampen the brush a little bit in the finishing oil and then pick up a little bit of the bone black with the tip of the brush. Then, to distribute the bone black throughout the oil, rub the bristles of the brush around a bit on a non-absorbent surface.
If the tip of the brush is too wet, pick up more bone black and distribute throughout the bristles again. The brush can also be dried by working the tip on a paper towel. Once you have achieved a thin, dry, dark finish, apply it by rubbing it around and working it into the low spots onto the stock. Repeat this process of loading the brush and applying to the stock until one very thin coat is on the stock.
Multiple applications since one is never enough. Layers will need to be built up to darken the stock.