This is a very easy thing to do so don't worry.
- If the plugin is downloaded from the Marketplace then go into your Engine install directory (mine is C:\Program Files\Epic Games\UE_4.24\Engine\Plugins\Marketplace). If it's download elsewhere then just go to step 2.
- Copy the plugin folder that you're wanting to add to your project into your project/Plugins directory
That's literally all that you need to do. Just open your project and done.
until next time