Welcome to Librarium Online!
What does the ink to do the model and why do you use it? I have been wondering ever since I saw some at my local warhammer store around a year ago...
Shading mainly, although it can be used to give a darker highlight (if your to lazy to mix paint) Use it where you want shading to be, it usually runs into the recesses, and you just wipe off what you dont like.
You can make highlights darker to, by using an ink close to the colour of the highlight (but slightly darker) and just paint the ink over it.