To use userscripts you need to first install a userscript manager. They come as extensions for various browsers:
- Violentmonkey - for Chrome, Edge, Firefox, Opera
- Tampermonkey - for Chrome, Edge, Safari, Firefox, Opera
After you have installed a userscript manager, head over to https://greasyfork.org/en/scripts/505396 and click on the green "Install this script" button there.