Drag & drop the RAR file onto Fluffy and check the box to enable it.
PAK
Put the extracted pak into your game directory (e.g. C:\Program Files (x86)\Steam\steamapps\common\MonsterHunterRise).
If you have multiple paks, they must be numbered sequentially, like re_chunk_000.pak.patch_002.pak, re_chunk_000.pak.patch_003.pak, and so on.
How to Uninstall
Fluffy Mod Manager
Disable the mod. You can then delete it from Fluffy using menu or deleting the folder.
PAK
Just delete the PAK file. If you have multiple pak files, make sure any and all mods installed after this one are re-numbered so that everything is once more numbered sequentially.