We'll start with a head that has an empty dark area where the hair will go later.

Here, I have loosely sketched some "curlers", little rolls, to show a basic pattern of curls. Tight curls often form little cylinders:

Now I use the "Flat Oil" painting brush (downloaded from https://assets.clip-studio.com/en-us/detail?id=1701386 ) to indicate the first curly shapes with a shade lighter than the dark base.

They sit on top of the little cylinders, like highlights.

Of course we have not only neat cylinders, but several individual strands as well.

 

Here's this step finished:

Now comes a very important step to make the head look 3D. Assuming a "natural" light source from above, I take a large dark brown airbrush and lightly spray it over the lower part of the head.

Now the upper part of the head is lighter, and the lower part is darker:

I also use a smaller dark airbrush to paint in areas of darkness.

Now we add more highlights! Focusing on the top of the head, I add several small highlights in an ochre yellow tone, using both the Flat Oil brush and the airbrush.

Be careful to use a color that isn't too bright.

Highlights that are too bright look unnatural if the light source is soft and "natural".

I add more and smaller highlights, making sure that the lower part of the hair has darker highlights than the upper part of the hair.

We don't want to lose the 3D structure of the head and hair as a whole.

These highlights are quite strong already, I'll exaggerate a bit for the sake of demonstration ...

Now I'm adding a new "Glow Dodge" layer and very lightly airbrush a golden yellow tone on it, on the upper part of the hair:

This way it acquires an otherworldly golden glow. Maybe not realistic, but pretty ;)

I cannot resist adding a glow to the lips and cheeks as well.

And done! :D