Save the zip somewhere to your pc, open Flagrum 1.2+, go to the Mod Manager tab, click "Install from ZIP", and select the 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 four 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