Think of this as the list version of the mod installation safety guide.

Before installing

Create a backup of your game folder. Write down your game version. Read the mod's README. Check community comments on that mod. Scan the download for malware.

During installing

Whenever you can, use a mod manager. Install only one mod at a time. Playtest after each installation.

After installing

Confirm the game boots. Confirm your save loads. Never go online with the modded install. Revert to your backup if there are issues.