Instructions
Jump and fly up to grab the first Star Coin out of the air above the third rotating section of ground if you have a propeller suit. If not, jump on one of the Koopas that come past; hit the "2" button just as you land on the Koopa's back to get a boost, which should allow you to reach the coin.
Fly across to the platform where the second Star Coin is, or use the Para-Koopas to get there. Alternatively, climb up the bean stalk, which you can reveal by hitting the middle block in the group of three just past the first Star Coin. You'll come back down on a platform in the air, within easy jumping distance of the second Star Coin.
Ride the revolving ground section immediately after the halfway flag until you almost fall off. Jump to the ledge to your left at the last moment, then walk toward the wall to reveal a secret area. Go down the pipe to a hidden room with a rapidly revolving ground and pipes that drop Goombas. Avoiding the Goombas, run forward to gain speed, then jump up to the red platform at the far right, where the final Star Coin is.