Instructions:
We're finally getting to the details. There's not really any 'trick' right here. What you do is make a new layer. You pick a normal, sharp edged round brush and you paint strands following the general shape of the locks you have. You use a dark colour to begin with. You pause, you smudge it a little with a spackled brush - still in the hair's direction... and then you paint new strands on top of it in a slightly lighter colour... pause, smudge... repeat. Keep picking colours from the hair underneath and make sure not to remove the general shapes you've already created - and since you're keeping all of this on a separate layer... feel free to erase away in places to bring forth the nice texturing you've already done underneath.

Because you've prepared and textured the hair so nicely already, there is no need to overdo this and you don't have to be so careful all over the place. The hair was already looking pretty nice before you got to this point.

Common mistakes:
Completely abandoning the shapes you've already set down. At this point, it's SO easy to fall back on the old 'every hair for itself' principle. These might be individual strands, but they SHOULD be moving in group. They should be together. Flowing in unison. One crossing over the other now and then and breaking each other up.