Including plugins: a definitive answer... please.

I’m using UI Navigation 3.0 in my project, and when I package it, on the title screen I get two Unreal error messages saying I need to include it as an additional directory. So I go to do that, but then it tells me I can’t include directories that aren’t within the project’s content files. So I copy the plugin’s entire folder from the marketplace folder in the engine’s contents and paste it into my project’s content folder.

Upon reopening the editor, it asks me to import all the new things I just added. All of the imports fail. Add the directory in packaging setting, under both assets to cook and non-asset to cook because it has assets in it but is also a plugin. I try to package the game and I get “unknown cook error”. I skim the output log and see it’s flagging all of the items in the pasted folder as duplicates because I’ve already installed the plugin into the editor.

But after looking at, like, 12 different, disparate answers (none of which answer my question) on the internet, I’ve gotten that I’m supposed to copy and paste the plugin into the content folder (and maybe also do something involving C++ but it isn’t clear if that’s only for a specific version of the editor or not). SO WHAT DO I DO?