From what I've been told, this tool was made for a really old version of skyrim and if used on the current version has horrible side effects. I could be mistaking it for a very similar mod, or it's possible it was updated recently, but I would double check before installing and using it either way!!
Mar 26, 2018 - Skyrim Ultimate Killer Mod allows the player to kill all the NPCs. Skyrim Npc Editor Finalizing A Cd. Geonox - Hair creation and Sims conversion.
It is a shame, as it's an awesome tool. I would love to use it to make some Khajiits more nice looking, as whoever made them on bethesda team really didn't know much about making 'animal-people' look attractive. XP And pretty much any thing having to do with a mod or tool not working, or not installing right belongs in this forum.
Basically, if you can't figure it out, it belongs in this forum! That tool is very old.
Does it still work? I read the posts on the mod page and it's full of people with 'not working this and that'. It does nto work well with custom races and custom body replacers. Anyway you need to select the plugins you wanna use in the NPC editor (to load) and then just click apply, that will bring you to the screenshot of 'how it's suppose to look like' I would however suggest just to use CK for this, as you can make 1 plugin to affect any number of NPC's to your desire.
(instead of 1 plugin per edited NPC) when done with CK editing you need to clean your plugin using tes5edit to remove the ITM records and you're good to go. It does not create plugins that work with the CK or TES5Edit because the author based it off of old TESVSnip source code that creates corrupt plugins. Once you make the plugin since you have all the files needed to create the NPC and if you really know everything there is to know about how plugins work it's possible to fix the plugin. However, you would have to go through hell to do it.
I have only gotten close to fixing one, I was never able to completely fix all the issues and make a reliable plugin.