If you would like to use an authenticator app for 2-step verification follow these steps.
Select "Security" from the drop-down menu.
Click on "2-Step Verification".
Click on "Authenticator app".
You can have multiple authenticator apps configured so choose a unique alias to distinguish between them. When you enter an alias click on the "Next" button.
Scan the QR code with your authenticator app. You may choose any of the many authenticator apps available. We choose Google's authenticator app.
After you scan the QR code you should see something similar to the above screen.
Enter the code you see in you authenticator app and click "Next" button.
You have successfully added an authenticator app. Click "Done" to go to Authenticator app configuration.