Place the Sword of Mana in the space directly to above Domina to create the Tree of Mana. Do not enter the Tree of Mana yet. Instead, enter Domina.
Go to the center of town and you'll see a duplicate of yourself doing bad stuff to a Sproutling. Go Right to the market. It will happen again. Go back to the center of town and talk to Teapo in the Lower Left door near the exit. Exit out of the building and follow your duplicate to the Outskirts of Domina. It will turn into Shadow Zero-One and you will need to defeat it.
Upon Completion: Return home, talk to Lil' Cactus, go downstairs (watch him write the Event on the leaf), and go back upstairs to save your game before continuing.