Before we start, download Facebook Authenticator App.

Log in to your Facebook account and go to Settings & privacy.

Choose Settings tab.

Choose Security and login tab, scroll down to Use two-factor authentication and click Edit.

Click on the button that reads Use Authenticator App. It will prompt you to confirm your Facebook password.

A pop-up window will appear for you to scan the QR Code with Authenticator App. Keep this window open as you reach for your phone. Note that anywhere (not just Facebook) that prompts Google Authenticator also supports Authenticator App.
Note: Do not scan the demo image shown below.

Take out your mobile device and open the Authenticator App. At the bottom of the screen, tap the plus button to scan a new account and hold your device up to the computer screen.

After successfully scanning the QR Code, the Authenticator App will automatically choose the Facebook logo and autofill account name and secret key. Confirm it’s correct by clicking Save.

Record the numerical token provided for Facebook. Note that this code refreshes every 30 seconds.

Returning to the Facebook website, enter the numerical token in the appropriate field. When done click Continue.

You will now be shown your Facebook back-up codes. These codes are one-time use each. If you run out of these codes, you can click the Set Up button again to re-roll a new set, but doing so will render the previous list obsolete, so be 100% sure to keep the latest set somewhere safe.