Right first things first, this is really a lot easier with a Wacom pen/pad, doing it with the mouse is possible but more time consuming. 1. Create a nwe file at 64x64 pixels, Click Airbrush and 2. pick a black colour, 3. the airbrush opacity to 100%, 4 put in some dots in the middle of the new file 5. zoom in if you want, this is basically gonna be the hair brush ;-)

OK we are gonna need three of these so create 2 more new files and including the one we just made, we have 1,2,3 at 64x64, 32x32, 16x16 pixels...now SELECT ALL on the first image 1. and click as shown here 4. and choose DEFINE BRUSH...woohoo a new brush appears, do the same for 2 and 3 creating three different sized brushes.....

OK now for each new brush 1. Double Click it and 2. set spacing to 1% and 3 click OK, this basically means that when you paint with these brushes you will drag several solid lines at once...

OK lets block in the hair, 1. Choose the airbrush and 2. Choose a darkish base colour, 3. create a new layer and choose a solid brush and 4. block in a rough estimation of the hair and after you have blocked it in 5. Click PRESERVE TRANSPARENCY, this basically masks off the hair so you can only paint inside the solid hair area...One other point is its well worth collecting some reference pics of hair off the net and studying what colours are used, highlights, shapes etc...

OK back to the airbrush, and 1. Choose the big hairbrush, 2. Tick pressure if your using a Wacom and 3. Choose a dark colour and start roughing in the lines of the hair, imagining the shape, flow, fall etc...

Ok after putting in some dark lines, now comes the magic...1. Airbrush (still on hairbrush) and 2 choose various lighter colours (even though it shows a dark one here ;-)) and dont forget to TICK-OFF PRESERVE TRANSPARENCY...3. Set the airbrush to SOFT LIGHT and around 50% pressure now put in some more lines lightly and start adding highlights to the areas you think will pick up light, this is the fun bit play around and keep working on it...

Remember when I said that was the fun bit? I was wrong this is :-) Right Choose the 1. SMUDGE TOOL and 2. choose a hairbrush 3. Set the pressure to 95% or thereabouts, now following the flow start smudging those scruffy lines around, now PRESERVE TRANSPARENCY is off we can paint outside the hair area, so drag some hair out around the edges etc to break up that solid line from when we blocked in the hair!

Time for adding some more highlights with a light colour and also picking out dark areas to define groups of hair a little, at this point its a case of adding colours in SOFT LIGHT (or other modes), smudging em about a bit and dodging and burning (see next step)......Rinse and Repeat...

Using the DODGE and BURN tool with the hairbrush is the idea, using it normal creates darker details and holding ALT creates highlight details, you can really get the details in using this and it only works on whats already painted, so no need to mask etc....Have FUN!