Conceptually, it's not that difficult. Decide where the light is coming from (the eyes, in your case), and put down 'light' where the light source would logically illuminate. The hard part is making this look natural, and airbrushes are actually fairly poor at this, because the paint isn't coming from the light source. It's super easy to over do.
One thing light sources do is that when you look at them, areas that would actually be much brighter than normal look dark because you're looking at a light source. So immediately around the eyes would look darker in the recesses right around the eye, while the cheekbones, nose and eyebrow area would be lit up.
I'm fairly new to doing
OSL myself, but I recommend keeping it subtle. Here's some drones I did a small amount of
OSL effect; I basically just painted a few lines around the light sources, though I was going for an overall 'force field' effect as well by edge-lighting areas. One thing I tried to do was give the light sources some bright points, but paint the
OSL areas in the basic light source color. That should make the light source more intense than the areas it's lighting up.
If you're going to try extensive
OSL with a brush, I'd say use fairly thin paints and pull the paint toward the light source (surface tension should keep most of the paint with the brush, so it's thinner the further away). Getting the paint just the right thinness will be key here, as you probably want it to be fairly transparent at a distance from the light source.