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:
Then navigate to the "Plugins" section on the left side of the Wordpress Dashboard.
You should now see the plugin you installed and need to click on "Activate" to make it work.
It takes a few extra seconds but is much more secure.
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