Back up before every mod install. Just do it. It's by far the most helpful thing you can do, and it only takes minutes.
What to back up
The game folder. Save files (often found in Documents). Config files. Steam Workshop files if used.
Cloud saves make this more complicated - some platforms auto-restore cloud saves and overwrite your local backups.
How to back up
Copy the folder to another drive or partition. Make sure the copy finished. Take note of the date and game version.
An external drive or separate disk is safer than the same disk - if that disk fails, you lose both copies.