How to install:
Use Vortex. It's easier.

Manual install:
Paste the .pak files in "C:\Program Files (x86)\Steam\steamapps\common\Ready Or Not\ReadyOrNot\Content\Paks"

If you downloaded Steam in a different directory and don't know how to find "\steamapps\common\",
go to Ready or Not in your steam library, click the gear icon near the right of your "Play Time" statistics, hover over "Manage" and click on "Browse Local Files."
This should bring you to "...Steam\steamapps\common\Ready Or Not"
Then just go to, "\ReadyOrNot\Content\Paks" and drop the .pak there.