First you will want to do some searching. You will need 1 photo of a person (animals work also), 1 photo of a skull or even a prosthetic mask and a texture to lay over all the images. Depending on the angle of your person's photo it might take awhile to find a skull that matches it.
Open all images separately, I usually use the face as my main background. Apply the skull photo and resize to match the persons face. After that with the skull layer selected either erase or use a vector mask to clean up the edges. Use a soft brush so it blends in more with the photo of your face.
Once your satisfied with the way the skull and your face look apply a layer of texture. Now comes the artistic part! Mess around with the settings. I used a linear burn for the texture to give it a dark creepy looking effect. I also desaturated both the skull and the face.