You have to use a Switchboard to navigate through this level, which is complicated by the numerous cannons and Parabones found across the whole level. You can pick up a Cannon Box near the first set of cannons on the Switchboard track to help even the odds.
Stamp
Early on, you see an platform with the Stamp off to the side of the Switchboard track, with ? Blocks next to it. Shoot the ? Blocks with a Cannon Box to make a bridge so that you can retrieve the Stamp.
Making it up this wall requires you to bounce from cannonball to cannonball. Fortunately, they all fire in sequence so the timing is very predictable. A Cat Power-Up, if available, can also help with this.
Green Star #1
After you climb the cannon wall, don’t go right away into the pipe. Check to the right side of the platform to find a breakable wall. Blast it open to locate the first Green Star.
Entering the pipee takes you to an underwater auto-scrolling section, filled with more cannons, blocks and ? Blocks.
Green Star #2
The Tanooki Power-Up, Cat Power-Up, and Rosalie’s Spin Attack can help reach the Green Star. Blast open the blocks pictured on this part of the swimming section to enter a hidden room with stacks of Coins and the second star.
A green + Clock is stashed away in the Clear Pipe near the second Green Star. It’s useful if you are running short on time.
The Switchboard tracks beyond the Checkpoint Flag need to be completed by shooting red ! Blocks with a Cannon Box, otherwise you’re doomed.
Green Star #3
The final Green Star can be reached only by changing the Switchboard track to head in that direction, and even then you can’t physically reach it without losing a life. Use the Cannon Box to shoot the star!
This final stretch can be rough. Parabones can ruin your shots on the ! Blocks to complete the tracks, while cannons continue to harass you.
If you’re not using a Cat Power-Up, you must use a long jump to bounce off a set of cannon balls to reach the top of the Goal Pole.
Rosalina and Peach have exceptional jumping capability, and do not need to use a long jump here.
World Star: Sprixie House
You can enter this Sprixie House the moment you arrive in World Star. It’s next to the Captain Toad level.