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The Head and Footer Scripts Inserter plugin's main selling point (It's actually free) is that it's intuitive and doesn't require you to edit any files. I like that because I'm not a Wordpress pro. I'm just a noob.
Okay, so after you click "Install now" and you've clicked "Activate" to activate the extension, you can go to Settings -> Script Inserter and that's where you'll begin using this plugin.