It's time to step to the beep again! The block platforms that appear switch after three ticks. Grab a Double Cherry and carefully cross the platforms. There are several Boomerang Bros in the area. Keep grabbing Double Cherries as you go. Go left and go to the blue platforms to reach the first Green Star. There's a safe block for you to use nearby. Nearby is a warp box.
The warp box takes you near the midpoint. On the back wall, you'll find 8-bit Luigi swimming. Go up the disappearing stairs and you'll find the midpoint. If you have 4 characters, you can stand on the platform to reach the stamp. Alternatively, you can climb up the wall if you're a cat. Go down the stairs for the warp box.
The warp box takes you near an area with a couple of Boomerang Bros. Go through the pipe and hit the P-Switch. Collect all the Silver Coins with your clones for the second Green Star. Ahead is a long slide made up of the switching platforms. Stay towards the center so you can cross easily. Try to keep as many clones as possible. At the bottom of the slide is a split P-Switch for the third Star Coin. Ahead is the flagpole, behind some Boomerang Bros and switching platforms.
The stamp is of the Double Cherry.