This star requires the boxes containing Wing Caps to be activated, so you need to go to the Wing Cap Course in the Castle Lobby.
But, there is one problem; you can’t go to the Wing Cap Course until you have at least 10 Power Stars – so first you’ll need to go to Whomp’s Fortress (The door with a “1” on its Star) and once you have 10 Power Stars and activate the Wing Caps you can come back here.
After Wing Caps Are Activated:
Once you have unlocked the Wing Caps go under the bridge and hit the red [!] box. It contains a Wing Cap. Grab it and when you triple jump you will start flying.
Go to the cannon near the beginning and shoot yourself up at the island in the sky (with the Wing Cap this should be easy). Once you are up there, hit the other red [!] box and jump into the cannon on the island.
Aim yourself at the 5 coin rings in the sky and launch yourself. You must fly through all 5 coin rings to have the Power Star appear but don’t worry you can take couple of shots at it. A good aim is the key.
***JMAN’S CANNON TIP*** The 4th ring is slightly higher than the others – so you may need to take two shots.
The Power Star appears on the platform directly below with the Goombas.