Drawing long hair is in many ways easier than short hair, as the 'surface' tends to be a little smoother, with broader sections of highlight, less dispersed than in short hair. Curly or frizzy hair can be more of a problem, but if you take your time and observe carefully, it is quite acheivable. You might need to ask your model to tame their hair a little with conditioner prior to sitting.

First, look carefully at the model or reference photo and identify the major sections of hair. Curls will often be rather flat, like ribbons curling back on themselves. Try to ignore odd strands at first, concentrating on the main bulk of the hair, and lightly outline the main shapes and sections.