Once logged in, click on your username/email in the top right corner of the site and from the drop-down select 2 Factor Authentication. From the 2 Factor Authentication page scan the QR code with a 2 Factor Authentication App (such as Google Authenticator) and then input your account password and the authentication code displayed in the Authentication App and press Enable 2FA.
Then each time you login you will first need to enter your username & password as normal and then on the following screen you will need to input the code from your 2 Factor Authentication App.