On this one, I used the vinegar and steel wool process. You can see the difference between the rosewood with lacquer applied (behind the nut slot on the headstock), and the fingerboard where I had applied the vinegar / steel wool solution.
You simply put vinegar Nd steel wool into a glass container, then wait!! Depending upon the strength of the vinegar, it can take between three / four days, and a couple of weeks to get the required darkening solution.
Applying the solution will darken the wood. You can also use varying solution strength to darken maple - going from "dirty wood" to fully black.