1. Outline The Major shape of the hair, and fill it in with the darkest hair color you are going to use...use a big/medium brush size for this.

2. Take a medium shade with a medium sized brush (30% opacity) and start from the hairline, and brush where u want the hair to go. Take the darker color u used, (30% opacity) and tone down the lighter color a bit. Reshape the hair if you have to.

3. Use a (medium/small brush, nothing tiny, opacity at 25%) start drawing in highlights, with a light color. Don't pick white linless you're doing a blonde person). Keep in mind the lighting in your picture.

4. Random strands of highlight/medium color. Tiny brush remodify hair shapre with a medium sized brush.

5. Add in shadow on the skin so its not like its floating on the head, you can go on refining...Im kinda lazy, and im not that great with hair, but this is a basic outline.