Go to https://mailcastr.com/ and click on the chrome store button. Wait for a second you can see a pop up asking add to chrome. click on it and extension will be installed automatically. once completed installation a new window will open asking you to log in. Use your preferred Gmail account to log in and that's it. Now your Gmail tab will be automatically refreshed to add mailcastr into it. You can see mailcastr icon just next to your profile pic on the gmail.