Face Off - Replacing face of one person with another's

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

Insert an image of a guy whose face will be put on the girl's body.

Press Ctrl+T. Rotate and scale down to adjust the angle of the face in accordance with the girl's body.
Click add layer mask button from the bottom of the layer palette.
Set black as foreground color. Select brush tool with a medium size settings and apply brush on the areas of the guy's face that are required to be knocked out such as his hair and neck.
Duplicate the layer and change the layer mode to screen.