I started out with putting some dye onto a piece of plasic, smear it out a bit and let it dry.
Then I would take a 0000 brush with short hairs and build-up the spots needed.
Removing dye is a pain, it is better to layer it and let it dry inbetween.
Use distilled water to wet your brush and dried dye.
Protect the part of the print that you are not working on with a piece of paper.

That is all I believe.