At the very start of the level, climb the tree on the left to claim Green Star #1. There is also a Sprixie atop the tree here, as well as on top of the right tree, if you wish to scope out the level. Continue across the bridge and you should see some moving blocks, one of which is a POW Block. The Red Pow Block sends out a shockwave that defeats nearby enemies, while also activating blocks. With this particular one, you can wait until they are on the right side before hitting the POW Block, which will also activate the ?-Block above the row, shooting out a power-up.
Start climbing the tower of ledges, stopping by first spot behind the one Piranha Plant, where you’ll find an invisible ?-Block with a 1-Up Mushroom. Continue climbing until you see a POW Block embedded in the wall. Time hitting the moving POW Block so it will trigger this one, revealing a pipe. In this new room is a P-Switch that spawns several blue coins. Collect them all quickly so you can unlock Green Star #2. Upon exiting that little bonus area, you should see the Stamp in between the two walls, so climb/wall jump to get it.
Continue to climb to the summit of the mountain, which is pretty uneventful, other than some Cat Goombas being in the way. Hit the Cat Wheel to raise a tower, climb it to the Cloud Cannon that launches you into the clouds. From here, hit the ? Block to spawn a Super Star then dash through the clouds defeating enemies and claiming Green Star #3 at the end. After exiting, climb back up to the cloud platform by the cannon, then dive to the right and climb that giant wall to the top to claim a 1-Up Mushroom. Cross the bridge and defeat the Mega Piranha Plant, then use the Warp Box to reach the Goal Pole. Note that the rightmost bell tree will yield a Super Mushroom for climbing to the top of it.