It's really quite simple, and can be summed up with a small explanation. But if it's so simple, why did I make a whole guide specifically for this achievement? Call it potty humour - you'll understand in a moment, if it hasn't already clicked.

Soon after having first started to upgrade your tavern, you'll unlock the outhouse. Yay, toilet duty. However, you won't even have to clean the loo to get this achievement.

Simply wait for someone to go out your backdoor after their meal - this can be any random guest, most drunkards, or most buff dudes (the ones whose orders are always urgent because they can't be bothered to wait 5 minutes in this economy). Once the person leaves for the outhouse, follow them.

Then, just stand outside their door, watching them through the moon window while they're trying to do their business. Just don't try this at home. And while you're at it, you can knock repeatedly on the door, because why not? All it'll cost you are a few points for being a rude host, but you'll quickly earn them back, and who's really the rude one when they leave the toilet all clogged up for you to clean, because the fairies won't come within 50 feet of a toilet mop?

Once you've had your fun with this, the achievement should already have triggered, and you can go back to doing... whatever you were doing with your tavern.