As soon as you begin the stage, you will have two Fire Bros in front of you, so you can either quickly defeat them or ignore them altogether. Open the screen door in the back, then ring the gong and go through the green pipe. Continue right and hit the ?-Block to spawn a baseball, which you can use to temporarily despawn the Blurkers behind the next sliding door. Go behind the wall here to find Key Coin #1 in the hidden portion. As you go up the stairs, two Hammer Bros will jump into action, although their hammers will likely break in the air, for some odd reason.
Collect Key Coin #2, then continue to the left, hitting the gong behind the Blurkers to gain passage to the left. Watch out for the Fire Bro on top of the Goomba Tower, whose defeat will make the next set of stairs appear. Open the sliding doors here and hit the gong, only to have a hidden room appear with Key Coin #3. Take a baseball up the stairs and throw it into the clear pipe to destroy the mine, then pass through it to get Green Star #1. To the right of that, open the sliding doors to find a Hammer Bro, some spikes on the ground, and Key Coin #4.
Pick up a baseball in front of the spikes and toss it to collect the Key Coin. As you continue right, you will find the locked Warp Box, as well as a clock pickup and two Boomerang Bros. Open the doors at the end to see three walls of Blurkers. Pick up a baseball and toss it at the middle wall, which is hiding a gong that spawns Key Coin #5 when hit. This will unlock the Warp Box, allowing you to move to an outside area. Collect a Propeller Box from one of the ?-Blocks, then trigger the Green Star Ring. You’ll need to dip down into the middle pit to collect some coins, but once you get all of them, you will be rewarded with Green Star #2.
When you journey to the upper portions of the roof here, you will see a gong on either side. The right one just spawns some coins when hit, but the left one will produce a Warp Box on some cloud platforms. Fly up to the Warp Box and take it to secure Green Star #3. With that, you can go all the way to the top of the castle here to find the Goal Pole, which will be flying back and forth. With the Propeller Box, you can fly above the flagpole and drop on the top of it when it passes.