Sharpie works great. I've pretty much switched away from oil lamp soot in favor of Sharpie mainly because it's just less messy than lamp soot. Your hands and fingers will stay much cleaner, as will your project.
My advice is to get a big fat black one. The red, green, and blue work also, but if you mess up and get it on exterior surface wood, it's a pain to clean out of the grain with acetone. Just use black, it'll blend better later on.
Paint several coats on at first letting it dry between coats, then rock on. Apply another whole coat each time you fit and tap the part.