When I started making Bees' Extra NPCs, getting the character from my game into a mod seemed like a ridiculously complicated task, and although there were guides available for similar goals, I couldn't quite find straightforward instructions for exactly what I wanted to do. I've written this guide for people like my past self who don't even know where to begin. It covers creating and exporting a head in Racemenu with vanilla assets, navigating the Character Gen Parts tab in Creation Kit, using NifSkope, replacing the vanilla file, and one way to create a pluginless replacer. I've also linked tutorials for setting up all the needed programs. (Many thanks to GamerPoets for his incredibly high quality videos.) It does not go over creating an NPC or making an NPC use assets from mods. 

If you have never used the Creation Kit, check out the page on creating an actor from the Creation Kit wiki(a very useful resource). 

All mods mentioned in the pdf are listed under requirements, but some may not be relevant to you. Please read the guide before you download anything.

If you're using assets from other mods, have a look at one of the following guides on Nexus instead:
How to Put Racemenu Created Faces on NPCs - A Guide by a Noob for Noobs
How to edit NPCs without Nifmerge

Let me know if anything is unclear and I'll do my best to help.

Thank you to smartco for their tips about making the face gen without using CK, and using Outfit Studio to fix the neck issues.