Two-factor authentication adds another step to the login process when you sign in to a website or app. The most common way to get these security codes is via a simple text message to your phone. Depending on the service, you can also get codes through specialized smartphone apps, via email, or through specialized “security token” devices. Here’s how to turn it on for six popular online services using two-factor authenticating for the most popular sites. For services like Gmail and Twitter that work with third-party client apps, you may need to generate app-specific passwords.”]