This level can be difficult as it requires quick reflexes to navigate through the pipes. There are lots of clear pipes, which is how you'll get around. There are Fire Piranha Plants that will breathe fireballs, sometimes into the clear pipes. Be careful. Kill one of the Fire Piranha Plants for a Fire Flower. This allows you to clear out the spike balls in the clear pipes. You can't go through parts of a clear pipe with spike balls. Up ahead is a looping pipe. To the left is a Starman. Use it to destroy the spike balls in the center so you can get the first Green Star. Leave the pipe to the right to reach the warp box. There are more spike balls in the pipes, but they can be cleared out with the POW Block. Up ahead are Fuzzies. You can't defeat them with fireballs, so avoid them. The second Green Star is in the top loop in a section of clear pipe. Once you get of the clear pipes, you'll reach the midpoint.
Ahead is a long clear pipe. Instead of going through it, jump on it and walk on top. You'll find the stamp up here. The pipe leads you to an area with more Fuzzies, spike balls, and POW Blocks. Use the POW Blocks to clear out the spike balls so you can go up to a moving cloud that takes you to the warp box. You'll end up near a Fire Piranha Plant and a clear pipe. Use fireballs (yours or the Fire Piranha Plant's) to clear the pipe. It takes you to a cloud cannon. Pick up a potted Piranha Plant and run. The potted Piranha Plant will eat the Piranha Plants in front of you and grab the third Green Star at the end. You'll drop back down, leaving the potted Piranha Plant behind.
Go down the slope with the Piranha Plants. Ahead is a clear pipe with two loops with a Key Coin out in front. There is a group of Fuzzies in each loop. With good timing, go through the pipe, then to the upper loop and to the right. Go through the right branch and get the two Key Coins. Go into the loop, going up when the coast is clear, then get the last two Key Coins in the left branch. Go down into the loop and to the warp box at the top. The warp box takes you to the flagpole. There's a clear pipe with a group of Fuzzies in it. Take the third exit from the bottom to reach the top of the flagpole.