Replacing face with a skull

Start by opening an image of a lady. We'll use her body but will replace her face by a skull.

Insert an image of a skull preferably having similar angle or facing direction.
Duplicate the layer and change the layer mode to color dodge.
Knockout the background of the skull. Scale down to the size of lady's face.

 

Adjust it on the face by squeezing it vertically.
Enable add vector mask from the bottom of the layer palette.
Select brush tool with soft brush settings. Take black as foreground color. Apply the brush on the facial area covered with hair so that hair appear over the skull.
Change the layer mode to multiply