First I do a quick outline of the subject in mechanical pencil, and if there are really intense/bright highlights in the hair I'll outline those. In this picture, there's only one really bright highlight on the top of her head.
Next I put down a light layer of 4B pencil and blend it, trying to make it as even as possible.
After I shade that first layer of graphite, I outline the spots where her hair is lighter. They're not necessarily all highlights, but areas of the hair that are light enough to where it would be difficult to lift out the dark pencil with a kneadable eraser and have it look right.
After outlining the lighter spots, I put down another layer of 4B to make it darker, and blend it. It's OK if, when you try this, you get smudge the outline of the white highlight a bit; you can lift it later, and you don't want it to be too crisp anyway.
Next, I use my kneadable eraser to pick out highlights and lighten up some spots that I might have smudged with the 6B. It helps if you smoosh it a lot in your hands to get it really soft, then pinch one side of it to make a flat, sharp edge so you can erase thin lines.
Now to make the hair look more realistic I get my 4B pencil as sharp as possible and draw a lot of thin lines throughout her hair to give the illusion of strands and locks. It's important to vary the darkness. The pencil strokes should be fast so the lines aren't too heavy, like this:
Remember that it's really important to follow the shape of the hair.
After I get done putting in the individual strands/locks, I go back through with my kneadable eraser and pick out more highlights and lighten up some spots I might have smudged.
To clean up the edges of the drawing and remove any smudges that might have gotten on other areas of the drawing, I use my retractable eraser.
As a last touch, to help make it more realistic again, I sharpen the 4B pencil and add thin whisks of hair around the head so it looks like little fly-aways.
I hope that helps! :) Take your time, because the longer you take and the more careful you are, the better it'll look. (Duh)