This is an underground level. Take the yellow pipe down, and you will find yourself in a cave. Above the first Pokey you find will be a platform and a pair of blocks. Hit the right block for a Super Leaf. For the next pokey, use the rising and falling platform to safely go over it. Then, jump onto the ferris wheel of platforms ahead. Be careful not to land on the one with a pokey! The same applies for jumping onwards. If necessary, jump on the other safe platform on the wheel before leaping forwards. At the end, you should find a safe platform with a pair of coin roulette blocks and a regular roulette block, each with a Spike Top crawling on it. Use the nearby spring to leap up and grab the coins and item when the Spike Tops are on the sides or top of them, and then touch the midpoint flag.
Next up is a pair of platforms, one above the other, with a pokey on the bottom one. You can simply jump above it to get to the pipe to the second part of the cave. However, you need to destroy or otherwise work around it to get the first star coin. To get said coin, hop on and grab the Buzzy Beetle to the top right, go down to the lower platform, wait for it to get low enough, and throw it at the POW block. You will uncover a second pipe. If you need to, leave the area and come back to respawn the Beetle. Inside the pipe will be a left and a right way down. Throw the nearby Beetle down the left passageway to clear it, and then hop down to get the first star coin. Then, proceed right.
When you exit the pipe, get ready to stomp on and grab the Beetle to your upper right. Then, climb onto the platform to your left, Beetle in hand. Shoot it down at the POW blocks to clear the way to the second star coin. Beware of goombas and the spinning shell along the way though!
Next up are a few pillars of platforms that go down or up endlessly. Get as far right and up as possible, and then make a leap to the right to land on a platform with a couple of Spike Tops on it. Keep on leaping from one high platform to another, and you will eventually reach the third star coin. If you take or fall to the lower route, you'll encounter some pokies that will try to get you while you're ferried on moving platforms. Either way, take the green pipe to go back to the surface. Then, jump on the platforms to the right in order to reach the flagpole.
Note: This level involves a lot of jumping over dangerous plants and using or avoiding shelled opponents. If any of these skills are difficult, you might want to practice on an earlier level to get some power-ups, lives, and valuable experience.