In the images of my daughter above left you see it would be hard to find the brightest areas, however by fiddling with the contrast and brightness they become more visible.
I find it best to lightly sketch an outline of where I am going to put these as I do my preliminary sketch. If you do the same remember you will need to remove the graphite before adding the ink or it will become permanent. I have experimented with various inks but always come back to Winsor & Newton as it seems to adapt itself well to being used as watercolour. It also widely available and reasonably priced.
If you are working from a digital photograph then there are various settings that can make this easier to see for the beginner or even a more advanced artist depending on the reference material. However this method is just as useful if you are painting from life.
In whichever device you are using you will find settings for brightness and contrast. By adjusting these you will find it easier to see the brightest areas for your highlights and the intensity required.
Personally I find turning up the contrast and reducing the brightness works well if I am struggling.
It does not damage my brushes anymore than ordinary watercolour so just clean with plenty of water and use your normal brushes.
The very brightest areas can be painted with neat ink, for a more gentle highlight water it slightly. I would suggest you experiment a little before diving in. It dries quicker than watercolour so a little speed is required. Soft edges can be obtained by using a damp brush alongside the ink to allow it to bleed slightly.
Below are a few examples of my work at this stage.