Login to Webmail with your current password,
Go to Preferences -> Accounts -> Primary Account Settings and then
Go to ‘Account Security’ above Password Recovery Account Settings:
and click on ‘Set up two-step authentication…’
Click ‘Begin Setup’ button
Enter your current password you use to login to Webmail and click on ‘Next’
You will now see the following popup window:
Download and install an authentication app for your smartphone or tablet. I found the Google Authentication app works well on my Android phone. Here is a list of tested authentication apps:
Once you have installed the authentication app click ‘Next’ to continue
Use the provided code to setup two-step authentication on your device’s authentication app.
When your authentication app is set up, click the ‘Next’ button and enter a 6-digit verification code from your authentication app into the ‘Code:’ field
Click the ‘Next’ button and you should now see a pop-up window to confirm you have set up two-step
Click on the ‘Finish’ button to complete the setup and activate two-step authentication for your Webmail account.
The next time you login to webmail, you will have to provide a 6-digit code from your authentication app, as well as your username and password.
If you click on ‘View’ under One-time Codes:
You will see a list of 10 spare codes you can use to login in case you loose your authentication app.