The level starts above ground in a small area with a single pipe and koopa. Jump into the nearby POW block to clear the way and defeat the enemy. Before going into the pipe, you may want to jump onto the ledge to the right, causing coins to appear. As you hit the bottom of a cave, you will hit another POW block, causing more coins to emerge. Continue advancing right, breaking the POW blocks to uncover a power-up. The first Star Coin is hidden on a platform with three koopas and two POW blocks. As one of Mario's larger forms, buttstomp into these blocks to reveal the path to the Star Coin.
Go down the nearby yellow pipe. Inside, you will find a P button. Press it and hurry along the course. Collecting coins is important, but the real goal is the second Star Coin. When you're done, either go back the way you came or enter the yellow pipe on the right to skip a portion of the level.
To the right, you should see a koopa walking near another POW block. Jump on it and knock it into the block, clearing the way to a special pipe. Shooting fireballs into the pipe nets coins. If you are not equipped with a Fireflower, use one of the blocks to the left. Eventually, a Mega Mushroom will pop out. Hurry up and grab it!
Keep on running to the right, through all the obstacles in your path, including the checkpoint flag. If you hurry, you'll reach a floating platform with the third and final star coin inside. Simply smash your way through in order to grab it.
From this point on, you can walk to the end of the level with no enemies in your path. Go up the blue pipe to return above ground. Hit the POW block to create a staircase, from which you can jump onto the flagpole.