On Tampermonkey:
Open the Options menu (right click on the icon at the top right of the browser)
Click the button that looks like a piece of paper.
Delete the other default code in the coding area.
Copy & Paste the code from Google Drive into the area.
Hit the save button (floppy disk).
It should come into effect.
If it's not working, be sure to ENABLE Tampermonkey (will show up when you LEFT click the icon).
If it's still not working, maybe you touched something or did something wrong.