Step 1: Open your Facebook App.
Authentication app
Step 2: If you don’t have an authentication app, go to install the DUO app section. If you have an authentication app move forward to the next section.
Step 1: Go to the App Store/Google Play.
Step 2: Download the recommended app by Facebook (DUO Mobile).
(Source: https://play.google.com/store/apps/details?id=com.duosecurity.duomobile&hl=iw&gl=US)
Step 3: Go back to Facebook and continue the process by following the instructions.
Step 4: Move to the next section.
Step 1: Copy the code.
Step 2: Enter the key to the authentication app you chose.
Step 3: Tap “Next”.
Step 4: The app will generate a 6-digit code. Once received, come back to Facebook, enter the code, and tap next.
Step 5: Facebook Two-Factor Authentication is on.
Step 6: You will then be directed to the Two-Factor Authentication page. Tap on “Recovery Codes”. Take a screenshot of them, print them, and keep them in a safe place.
Step 1: Tap on “Text Message” and then “Continue”.
Step 2: Enter Your Phone Number and tap “Next”.
Step 3: Enter the 6-digit code and tap next.
Step 4: Enter your Password and tap next.
Step 5: Facebook Two-Factor Authentication is on.
Step 6: You will then be directed to the Two-Factor Authentication page. Tap on “Recovery Codes”. Take a screenshot of them, print them, and keep them in a safe place.
Source: Facebook.com