Next, click on the Install button. Make sure you're okay with the extension accessing your data on Facebook.
You should get a notification telling you that the extension has been installed.
Open a new tab (or restart Chrome) and head to Facebook. Click on the drop-down menu in the top right corner, and choose FFChecker Interval.
Here you can change how frequently FFChecker checks your list of friends for changes. The default is 2 hours, but I would set it to at least 24 hours if I were you.
You're done! Now all you have to do is wait till someone unfriends you on Facebook.
See also: How to check who's unfriended you on Facebook by Emil Protalinski