A word of precaution; testing in production is not to be confused with releasing untested code.

It starts with acknowledgement that all serious problems are discovered in production and occurred because unforeseen things happened.

Deliberately go to your production environment and do unforeseen things like turn off a server, kill a process, pour coffee on your keyboard, upgrade a service during high-load.

If your system is built to survive it, then it should! You’re only sure it will if you (dare) test it.