The first Star Coin is in plain site about a small fast rotating circle of land. Use the flying Koopas as a boost to get it.

The Secret powerup exit will land you right next to the second Star Coin. Simply jump over to the small platform its on. If you are trying to get this from below use the flying Koopas to get Mario enough height to reach the platform.

When you hit the Midway Flag you should notice the telltale indentation of a hidden area below you. Use the momentum of the rotating circle to land yourself in this hidden area. Take the pipe down to the secret area. This area is a FAST moving rock circle mass with Goombas constantly falling from the pipes on the right. Work your way to the left platform to get the Star Coin and then exit the area FAST to keep from getting damaged.