Go to your install folder and navigate to \Stalker2\Content\Paks

Create a new folder there and rename it ~mods

Place your .pak files in that folder

You can also place the ".pak" files in their own folders within the "Stalker2\Content\Paks\~mods\" directory to keep it a little more tidy.

e.g. '\Stalker2\Content\Paks\~mods\RemoveVignette\z_RemoveVignette_P.pak'

Once you have the "\~mods\" directory created, you may want to use a mod manager like Vortex or M.O. They should take care of all of that and allow you to enable and disable them making it easier to troubleshoot or update them.