The first thing you need is your picture

As with every one of my tutorials, I advise you make a duplicate layer for every step.

The first thing you need is your picture.

You need to extract your subject from the background by using Filter > Extract...

Use the smart highlighting option, and draw around the subject, not worrying too much about getting every strand of hair. Once you have drawn your outline, fill the rest of your subject and press preview.

Use the edge touchup and cleanup tool to remove and rebuild any unwanted or missing edges. Once you are happy with your cut-out press ok and you are brought back to your new image.

Next you need to take the Smudge Tool and change your brush size to very small. Two or three px at the most as this is the tool that you are going to use to create new hair. Next you need to set the strength. Ther higher the strength, the longer the strands of hair will be. So use your own judgement here. I used 92%.

All you do now is follow the direction of the hair and create new strands flicking out from your cut-out. eventually over time you will create a natural looking hair.

If you were doing shorrt male hair for example you may want much shorter strands, and so you need to reduce the strength of the smudge brush.

And here is your result! Now you can place your subject on any backdrop or setting you wish.