1. First you need to download your desired plugin, you can find many plugins on the spigotmc website.
2. Open your server's files and go to the "Plugins" folder.
3. Upload your plugin and restart your server.

The plugin should now be working, you can view all your plugins with the "/plugins" command. If a plugin is listed with red color, it is not working. Look at your server logs to find out why it is not working.

Common reasons why plugins do not work
Plugins sometimes don't work because either the plugin is not supported for your current server version, or the plugin requires an additional plugin to work. Always check the description of the plugin to find out exactly how to install the chosen plugin.