A friend just showed me this, and I’m in awe.
Start with the airbrush, and use these settings:
The most important change is highlighted in red. For finer, more wispy hair, set the number lower. For thicker strands, set the number higher. Try to use quick, even strokes, and make sure you layer different shades to get a more realistic effect.
You can play with the other settings to get different effects as well. Enjoy!