If you haven’t already, complete the SMS/Voice setup and enroll your account in 2-Step Verification using your phone number. You can add the Google Authenticator app only after you’ve already enrolled using your phone number.
On your mobile device, open the Google Authenticator application.
Tap the plus icon.
To link your mobile device to your account:
Using Barcode: Tap "Scan Barcode" and then point your camera at the QR code on your computer screen.
Using Manual Entry: Tap "Manual Entry" and enter the email address of your Google Account. Then, enter the secret key on your computer screen into the box next to Key and tap "Done."
To test that the application is working, enter the verification code on your mobile device into the box on your computer next to Code, then click "Verify." The clock icon on your mobile device will let you know how much time is left before the verification code expires and a new one is generated.
If your code is correct, you will see a confirmation message. Click "Save" to confirm. If your code is incorrect, try generating a new verification code on your mobile device, then entering it on your computer.