These golden rules are not my own, but the recommendations from the online documentation listed above.
For best results follow them!
1. Do as little as possible—the default configuration should work as is.
2. Do not edit the default configuration files unless you understand what they do.
3. When you make changes, keep them small and make backups.
4. Confirm that the changes work as intended by running FreeRADIUS in debug mode and carefully observing the output during various scenarios.