Things You'll Need
Instructions
Acquire the Paper Airplane ability.
Talk to the spirit trapped in the Black Chest. The spirit can be found in the Rogueport Sewers once you pass through several pipes. Hit the blue exclamation block. A stairwell will appear. Climb the stairs and enter the doorway at the top. Talk to the chest inside. The spirit will ask you to assist him.
Find the Black Key. Exit the doorway, then turn left to an airplane launching platform. Jump from the back edge of the platform to retrieve the key hidden just beneath.
Return to the Black Chest found in step one. To do this, fall from the key platform. Next, climb the previously revealed stairs and enter the doorway. Talk to the chest a second time, and agree to release the spirit inside.
Fly as a Paper Airplane
Stand on an airplane launch platform. The easiest one to find is the one just above the black key you retrieved earlier.
Press the "Y" button to transform Mario into a paper airplane. This will immediately set him flying in the direction that the launch platform is pointing.
Control the airplane by pressing "forward" and "back" on the analog control stick. "Forward" will drive the plane downards. "Back" will point the plane's nose upwards.
Create longer flights by pressing "back" whenever the plane starts to sink, and "forward" whenever the plane starts to rise.
For an example of how to fly really far with the plan, watch the linked video below.