Make sure you disable any other slither.io extension and scripts first!
How to install user scripts?
Firefox and related browsers: Greasemonkey.
Google Chrome, Chromium, and related browsers: Tampermonkey.
Opera (version 15 and later): Tampermonkey or Violentmonkey.
Opera version 12 and earlier supports user scripts natively. Violentmonkey provides a friendlier UI and better compatibility.