Activating FreshInbox is as easy as a pie. To activate FreshInbox:
Open or navigate to mail.google.com
Click on "Sign in with google" button
Choose an account from Google Accounts Selector
Click on "Allow" button for providing Freshinbox with the necessary permissions.
Click on "Let's get started" button to start using Freshinbox
If you want to install Freshinbox at a later point in time, choose "Show later" button.
To activate Freshinbox later, you can click on the Freshinbox icon present on the upper right corner near the search bar. 
After clicking the icon, follow the steps above to activate Freshinbox.