First I create a new layer, then I take a red, 1px brush and draw out the shape I want for the hair. Next create a new layer for the base of the hair (put it under the first layer you created), pick the 75% hardness brush and select the base color, set the brush to "Fade Tapering", make sure to set it on reverse (brush size & length will vary). After you have your base done delete or hide the first layer that you created and start on the highlights/lowlights, select about 4-5 colors for this step. After you're finished with the highlights/lowlights merge all the hair layers and adjust the hue and saturation until you get the right color.

I hope this helps, I'm not too good at explaining stuff like this. lol