Companion to the mod installation safety guide. This is the list version.

Before installing

Back up the game folder. Note the current game version. Read the mod's README. Check community comments for the specific mod. Run a malware scan on the download.

During installing

Use a mod manager when possible. Install one mod at a time. Test the game after each install.

After installing

Verify the game launches. Verify your save loads. Don't take the modded install online. If anything breaks, restore from backup.