Photoshop. Tracing Around Hair.

Open the back ground photo in Photoshop

Place the foreground photo in a layer above it.

Use the Quick Selection Tool to select girl, her body, and her hair as best as you can.

Click on Refine Edge at top of screen.

Turn on Smart Radius Set to 5

Select Refine Radius tool

Paint on the areas which are grey to delete all grey

Click Decontaminate Colours

Adjust Amount  65%

Output to New Layer with layer Mask.

If required select the Burn Tool and darken up the hair strands.

An alternate way to achieve this is to use the Background Eraser Tool (E)

With the picture of the girl in front, select the Background Eraser Tool.

Set the Control Panel to these settings

Sampling: Once

Limits: Contiguous

Tolerance: 50%

Protect Foreground Colour : Unchecked

Now, simply left click on the grey area ( to tell the tool this is the colour you wish to delete) and holding down the mouse button, paint out the areas you want to delete.

Once again, use the Burn Tool if necessary.