I use India ink wth whatever finish I feel like using over the top. Just let the ink dry for atleast 24 hours, longer is better or it could smear a bit. With tru oil, I'll put a coat or two of tru oil on to make the surface smooth and the ink doesn't run in the wood grain. Then I put a few more coats over the top with the rest of the bow. I'm still trying to figure out the best method to reduce runs when I use wax/grease finish. At the moment I just polish the wood as smooth as I can get it where I want the marking.
Kyle