The hair is a mass, it has a volume, that is the most important. Start by painting the dark brown mass around her head first. Look out for the darkest part around the neck! That part makes up some of the volume. Use the paintbrush with some stiffness and paint the light brown strands, not too many. Add highlites the same way, using the stiff brush. The hilites make volume, they are often on the top of the hair, but that depends of the lighting, of course. Lastly you can add details, individual hairs here and there. This is a painterly approach. If you like manga, have alook at the manga hair painting/drawing technique.
I hope this is this of some help.