Instructions
Get to Level 6 by raising your Pinatas and tending to your garden. Jelis seem to appear more often when you are exactly a Level 6 gardener. They will not appear before Level 6.
Wait for night to arrive. If possible, remove or sell all of the artificial light sources in the garden. The Jeli loves the dark, so it is more likely to visit you if the area is completely light-free.
Watch your garden carefully throughout the night. Unlike other Pinatas, when a Jeli enters the garden, you will not receive any kind of notification.
Take out your shovel when you see a Jeli come into the garden. Smack the Jeli with the shovel once. This domesticates it, and it becomes a resident of the garden.