Green Star #1
You can see the first Green Star atop a Goomba Tower at the very start of the level. It’s hard to miss, and it’s important that you retrieve it quickly: This level starts you off with 30 game seconds on the clock.
Don’t waste time diving into first set of Clear Pipes! Just run along the top of the Clear Pipes and run past the Fire Bro. This way you avoid the fireballs being thrown into the Clear Pipes. The second set of Clear Pipes might be worth the risk, as they contain a + Clock.
Ground pound the ? Block closest to the Goomba Tower topped by a Fire Bro to make a tower tall enough for you to take them all out. The Fire Bro must be eliminated for the Warp Box to the next section to appear.
Green Star #2
You can reach the second Green Star by revealing a set of invisible ? Blocks next to the Thwomps here, and then jumping up to the top of the wall. You can also ride the Thwomp if you prefer. Either way, run along the top of the wall back towards the start of the section to find the star. If you’ve got a Cat Power-Up, this can be done even faster.
Green Star #3
After crossing the Thwomp section, take one of the two Clear Pipes over to the next set of platforms (or jump across the blue lava, if you’re using a Tanooki Power-Up or Princess Peach). Ground pound the glowing spot near the Red POW Block platforms to reveal the third Green Star.
Before you can even reach the Goal Pole, you’ve got to take out the Fire Bro on this set of platforms. Defeating him will also take out the Goomba Tower and summon a Warp Box for you to use. Once you reach the Goal Pole zone, if you’re lacking a Cat Power-Up, you’ll have to use a Fire Bro to jump high enough to the top of the pole.