Drop the .jar into your plugin folder on your server (make sure the version is supported)
Reload the server (/reload confirm)
Default Rank:
The default rank that players that join will get is set to be the rank with name "default" in the config. If this rank does not exist on load, the plugin will generate a "default" rank. This can be customized just like a normal rank, and is assigned to be the default rank for a player who does not have a rank.