At the very start of the level climb the tree on the left to claim Green Star #1 . Continue across the bridge and work your way to the Red Pow Block that will take out the Piranha Plant. Collect the Super Bell from the ? Block then look right of the Pow Block for a hidden 1-Up Mushroom in a block. Climb up passed another Pow Block to spot the Stamp between two ledges.
Just past this section at the left end is another Pow Block . Hitting this and the next one will open up a path to 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 .
Continue to climb to the summit of the mountain. With the Cat Power-Up, climb up the large wall to your right just before the bridge to claim 1-Up Mushroom then jump across to the left. 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.
Defeat the Mega Piranha Plant after you cross the bridge and use the Warp Box to reach the Goal Pole.