Choose either manual installation or install via Flagrum.

Manual Installation:
Copy the files from "Manual" zip to the respective folders in your FFXV directory. Back up your originals first.

You don't have to install everything. If you only want to fix the neck seam on official 4K attires such as the Miqo'te outfit seen in the screenshot, the first folder below is enough (younger Noct, 4K).

If you want to use (my) Steam Workshop mods showing a lot of skin, then I'd recommend installing both younger and older Noct's textures and selecting 4K or non-4K depending on whether you have the 4K pack.

Folder "datas\character\nh\nh00\model_000\highimages" is for younger Noct, 4K textures.
Folder "datas\character\nh\nh00\model_000\sourceimages" is for yournger Noct, non-4K.
Folder "datas\character\nh\nh00\model_100\highimages" is for older Noct, 4K textures.
Folder "datas\character\nh\nh00\model_100\sourceimages" is for older Noct, non-4K.

Flagrum Installation:
Save the "Flagrum" zip somewhere to your pc, open Flagrum 1.2+, go to the Mod Manager tab, click "Install from ZIP", and select the "Flagrum" zip you just saved. Flagrum will do the rest for you. It will even back up your original files for you. If you want to edit the mod, eg delete unnecessary parts, you can do that yourself via Flagrum too.

In case you do not have the 4K pack installed, Flagrum should install the mod as inactive/disabled by default. Find it in Flagrum, delete from it the two entries that reference the "highimages" folders (these are for 4K), then save your changes. Now you should be able to activate the mod without Flagrum crashing.

Get Flagrum: https://github.com/Kizari/Flagrum/releases
Join Exineris, the support discord server: https://discord.gg/7cNNwwJKsJ