1. Preparing your image.Adjust and edit your image up until the point where you will add the hair. This prevents having to match lighting and color later.
Create a new layer and name it Colors. Choose the color of your hair and pick 4-5 tones going from the darkest to the lightest. Use a regular hard brush to paint samples of them on this layer. This will just make it easier to sample from them later.
2. Creating the base.Create a new layer and name it Hair base. Sample a medium tone and use a 20 px hard air brush at 100% opacity to paint the base and shape of the hair. Use a lighter tone to go over the hand and delineate its shape.
3. Painting the hair.Create a new layer and name it Hair strands. Change your brush to a 2 px soft airbrush at 40% opacity. Sample a light tone and start painting strands of hair following the shape and direction of the base and hand until you've covered all areas. Vary the brush size between 1-2 px to give it a more natural look.
Repeat using darker and lighter tones (except the lightest) until you're happy with the coverage.
This is just to give depth to the hair itself so don't worry too much about the contour shadows and highlights, we will adjust that next.
4. Applying shadows and highlights.Select the Hair strands layer. Use the burn tool with a soft brush at 30% exposure to accentuate the shadows and create depth where needed. Select the dodge tool and use the same soft brush at 10% exposure to add the highlights.
5. Finishing touches.Create a new layer and name it Loose hairs. Select the same brush you used to paint the strands. Sample the lightest tone and paint loose strands all over the hair, making sure they follow the natural direction of the piece. Sample a dark tone and do the same all around the outer edges.
Discard or hide the Colors layer.
And here's the final image. I hope you find it useful.
Some stuff