This star will involve you using the cannon to uncover a star that is hidden from view. If you haven’t unlocked the cannon for use yet, jump up to the area with water and head to the far side, where both the Bob-Omb Buddy and cannon are located. Get in the cannon and the default aim will be to the right of the large flagpole. To the right of that will be the rotating bridge and the Red Coin you see floating in the air. Directly underneath the Red Coin, to the right a little bit, will be a sharp corner of the stage.
This is the section you will be shooting at with the cannon, but you will need to adjust your aim a little higher than it. You’re aiming for the corner portion of the stage there and if done right, the corner will explode and reveal the star. All you have to do now is head up to where the rotating bridge is located and nab the star yourself.