Go to Profile (by clicking onmenu on the left corner):
Under Google Two-Factor Authentication click on the Enable button.
Download and open the Authenticator app on your mobile device.
Click "Add new account" (or the "+" plus sign) in your authenticator app.
Using the Authenticator app, scan the QR code shown on your security settings page (or enter the secret key available on your security settings page). This links your mobile device to your account.
Enter the code that appears in your app.
Write down your 2FA recovery code and keep it somewhere safe. This will help you if you lose access to your device with the 2FA app or change the device.