When two-factor authentication is on, after you enter your password (or sign in with Google) you will see the “Two-factor authentication” screen.

1. Open your authenticator app and find the current 6-digit Coldout code.
2. Type it into the "6-digit code" field.
3. Click "Log in".

Codes refresh every 30 seconds, so if one is rejected, wait for the next code and try again. Make sure your phone’s clock is set automatically, since a wrong time is the most common cause.