Find a time when there aren't any/much people on and select the Yes, run update/validate option under Steam Update. Optionally, you can also wipe your map and blueprints.
You literally just drag and drop the plugin into the server/experimental/oxide/plugins folder. Make sure that you do indeed have oxide install. Also, make sure that your Oxide version matches the current release and is compatible with the current version of rust. May I ask what plugins you are trying to install?