Properly, you need a good quality black ink...any black ink, including anything made in China, etc. But, if you must have India Ink, you may pay more...check it out. Then, go to Wally's and buy 350 ml of their pharmacy isopropyl alcohol (70%), and add maybe two tsp of the ink to that. Then, using a discarded hairpsray mister or something, half fill it with water and fill the rest with the ink/alcohol mix. That new mixture should do a good job of gentle details highlighting and staining. The original mix with the two tsp of ink is for heavier, quicker work.
-Crandell