Instructions
Play through the game until you're a level 6 gardener. Wait for it to become night. It should be roughly 2 in the morning for the best chance of a Jeli appearance.
Turn off all the lights, like the Firebrand, that are active in the garden. A Jeli won't appear unless the garden is in total darkness.
Watch for the Jeli to appear. You receive a notification when most new Pinatas enter the garden, but no notice is given for the Jeli. The Jeli looks like a large purple and pink ball of fur, and it's wearing a dark purple shirt.