Simply complete 20 factory requests.
From time to time, you'll get a pop-up telling you that they're looking for a pinata. Sometimes it'll be a specific pinata, and sometimes it can be up to you. All you have to do is accept the challenge, place the crate somewhere in your garden, and load the specified pinata into the crate.
After a short while, your pinata will be returned along with some free candy.
These do not have to be consecutive. You can complete some and skip a few and still get this achievement.