By downloading Right Inbox for Gmail below, you agree to the Terms and Conditions of use.
Click Allow to confirm at the top-left corner of the window.
Another popup will ask for your confirmation. Click Install Now to finish installation.
Click Restart Now button to put the changes into effect.
You are all set with your browser. Now you can go to Gmail or just refresh the page if you already have it open and complete necessary steps for authentication when prompted.