Hold right. As soon as you touch the ground, jump, then begin running. This will make sure the Waddle Doo up ahead will walk forwards.

Jump over the Waddle Dee and land on the edge of the platform. Slide off the edge and inhale both enemies to get a double star, then begin running.

If the Waddle Doo up ahead does not jump, you should probably just restart. If it does jump, jump towards it and shoot out the star a bit below it. Your trying to get a star to hit the Waddle Doo, Bronto Bird and Sparky. Start running again as soon as you hit the ground.

Jump onto the platform, run forwards, jump between the upcoming enemies. Then do a full jump and puff and little as possible to get on the higher ledge.

Slide off the edge and ram into the Hot Head. You should defeat and bounce off of it to fly towards the door. Enter the door as you fly towards it.