After you set up two-step authentication, to log in to your Shopify admin, you must use the authentication method in addition to your email and password. If you've set up backup authentication methods, you can choose which method to use when you log in.
Steps:
From your Shopify adminlogin page, enter your email address and your password.
ClickLog in.
Optional: If you set up more than one authentication method, then click the method that you want to use.
Authenticate using the method that you set up to perform two-step authentication.