This is the most secure way since we don't know if using the built-in ftp method of Wordpress is secure (that is do they store, record or transmit the ftp login information to a third party like the NSA?).
Upload or wget the plugin and then unzip it from the Wordpress website tot he following path of your wordpress:
/yourwordpressinstall/wp-content/plugins
Then navigate to the "Plugins" section on the left side o........