I'm looking to do what I used to do in Morrowind - replacing NPC faces with celebrities, friends and family members. It was easy to do - you needed the NPC's original face texture, your new desired face texture, and in photoshop you combine them, morphing your new texture as needed to fit the previous facial features and making it fully opaque. Easier to do than to explain.

I don't know how to do it in Skyrim.

I've seen mods that have replaced NPC/companion faces. Some are excellent, like this Jessica Alba mod (which is obviously texture replacement or addition, not sliders):

http://i.imgur.com/wUZXMMs.jpg

but how did they do it? And why is it difficult to google a tutorial for it? All the results I'm getting are completed mods, not tutorials.