Before any mod install, back up. This is the single most useful precaution and it takes minutes.
What to back up
The game folder. Save files (often in Documents). Configuration files. Steam Workshop downloads if applicable.
Cloud saves complicate this - some platforms restore cloud saves automatically and overwrite local backups.
How to back up
Copy the folder to a separate drive or partition. Verify the copy completed. Note the date and game version.
An external drive or a separate disk is safer than the same disk - if the disk fails, both copies go.