Pop down to the 2-step verification section and flip it on by clicking the big Settings button and following Google's step-by-step guide to link your account with the number of a cell phone or land line. Google will either text or robocall you at that number to provide a six-digit code every time you try to log into your Google Account from an "untrusted" device, so make sure you use the number of a phone you keep close at hand.
You can also generate one-time use codes that you can write down and save for times when you want to log into Google in the absence of cell service. Generate five or ten of these codes, and keep them in your wallet for emergencies. Also consider downloading the Google Authenticator app for iOS and Android if you don't want Google sending you text messages every time you check your mail from a new computer.
It's simple to use, and can generate codes for any authentication service that employs the TOTP (Time-based One-Time Password) algorithm, including Facebook and Dropbox. I recommend setting it up to do so if you're going to be enabling two-factor authentication on other services, but be aware that this will make it very difficult for you to log into those services without your phone. If your phone is lost or stolen--or if you just accidentally delete the authenticator app--it's possible to download a fresh version and re-authenticate across every service you use, but it's a real pain.
Facebook is easy by comparison
Facebook was a little late to the two-factor party, but at least it took the extra time to smooth out the setup process. Enabling two-factor authentication for your Facebook account is a snap. Just log into Facebook and click on the blue gear icon in the top-right corner, then click on your Account Settings menu.
Next, select the Security section from the navigation bar on the left-hand side of the screen, and switch on the Login Approvals feature (Facebook's term for two-factor authentication) by clicking the appropriate checkbox. Facebook will walk you through the process from here, explaining how to receive and type in a unique alphanumeric code every time you want to access your account.
To get that code you'll need to either download a mobile authenticator app that generates codes every time you log in, or give Facebook your cellphone number so it can send you authentication codes via SMS. I recommend going the authenticator app route. It's simple to use, and you don't have to wait for Facebook's servers to text you your code. Plus, you can also add a cellphone number as an additional backup if the app fails to work.
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