When Ghostly Dancers enters the battlefield as an enchantment due to Secret Arcade, its first and second abilities trigger.
Resolve the second trigger, creating a 3/1 Spirit creature token.
When the Spirit enters the battlefield as an enchantment due to Secret Arcade, Ghostly Dancers' second ability triggers, creating a 3/1 Spirit creature token.
Repeat step 4.
Notes
This combo creates a mandatory infinite loop at step 4 that will cause the game to end in a draw if it is not interacted with. It is highly recommended to only begin the combo if you have a way to break it.