How do I turn on two-factor sign-in?

Click your name or picture at the top right (on a narrow screen, the menu button), then My Account. On the My profile page, find the Two-Factor Authentication card and click Enable Two-Factor (if your email address is not confirmed yet, confirm it first). Scan the QR code with an authenticator app on your phone, type the code the app shows into Verification Code and click Confirm Setup. Save the eight recovery codes: they are shown once, and without the phone and the codes there is no way to reset two-factor yourself. Set up two-factor authentication has the rest.