First you need to get your 2 images you want to combine.
Take off the background on your skull.
Then make the skull line up the with the face.
Then erase part of skull to show the face.
Change the color so they look alike and make it look nice.
Erase the background.
You don't need to but you should add a white background.
Then make a copy of it and put it to the side.
Then warp it so it spreads out.
Then add a layer mask and paint it black.
After that you get a white brush and you
paint little spots.
Erase the other layer in little spots.
Then change the lighting and color of the image until it looks good.
Then your done.