How to port a head from another source into Skyrim?

[xiwibik30]

Let's say I have a desired head that I want to port to Skyrim. There are similar mods that do the same thing, such as COTR that adds a head from Daz, zzjay's mods that add characters from The Witcher 3, Erin Race that adds an anime head, and a few more. 
My question is, how exactly are such mods created? How do I accurately fit the neck from the desired head to the Skyrim body? If I just use Vertex select in Edit mode or sculpting to fit, there is a noticeable seam. Also, how do I accurately edit UV of the desired head to match the default head's UV so that the textures are compatible? 

 

I know I'll also need to create a custom race and .tri files, but at the beginning I'm only interested in neatly matching the head to the body.

 

I'm using Blender

[ghastley]

Since the standard head fits the body with no seam, I would think that the first step is to import that, retain at least one row of faces, and blend the new head to the top of that. Keeping faces, rather than just the seam vertices, means that the normals at the seam are also kept.

 

That also keeps a part of the texture mapping. You have the problem of merging the old and new textures, but at least you know where the seam part comes from.