Stamp: Shy Sprixie
There's a clear pipe on the lower end of the first cloud on the map. Use it to reach the tent, where you'll find another stamp inside.
World 6-1: Clear Pipe Cruise
When you reach the first set of pipes with some spikes inside, use the outer loop of the first set to collect the Invincibility Star. Use it to safely move through the spikes on the left side of the pipe series.
When your reach the pipes with groups of Fuzzies moving through them, you'll spot a star in the second pipeline. Time your entry to avoid the Fuzzies, then loop around the top to grab it.
Stamp: Captain Toad
Just after the checkpoint is a long clear pipe. Instead of riding, hop on top. Carefully run along it and jump to the higher section to find this well hidden stamp. You'll need a Cat Suit or a spin jump to reach this area.
After scaling the large vertical wall, you'll arrive at pipe with some spikes inside. If you have a Fire Flower, you can toss a fireball to destroy them, the hop in to reach the cloud above. This will launch you to a secret area, where a star lies at the end of the course.
World 6-2: Spooky Seasick Wreck
When you come to the pair of Bullies, take them both out by knocking them off of the edge of the course. You'll get a star for slaying them both.
Stamp: Fire Flower
After entering the wreck, you'll see this above a platform with some circling ghosts. Use the changing tide while avoiding the ghosts to reach the platform and leap for the stamp.
Just a little further on are two question mark bricks. Hit the one on the right and grab the Invincibility Star that emerges. Use this to safely take out the circling ghosts below - there's a star inside the hole they where guarding.
This one's actually pretty easy to grab if you move carefully along the planks past the checkpoint. Watch out for the few Boos that spawn too!
World 6-B: Fire Bros. Hideout #3
There's a spot you can examine near World 6-2. Do so to reveal a warp pipe that takes you to World 6-B. Here, take out the pair of Hammer Bros., pair of Boomerang Bros., and single Fire Bro. before time runs out to earn a star.
World 6-3: Hands-On Hall
When your each the first large set of doors past the Bowser gong, tap them open. Now tap the seemingly blank wall behind them to reveal some coins, two Galoombas, and a star.
When you reach the roof, you'll spot this floating over empty space. Head to the right side of the roof and hit the question mark bricks to discover that they actually Propeller Blocks. Grab one and use it to drop to the star, then safely propel back up.
You could propel to the roof and finish the course early now. Instead, collect all the keys to unlock the warp box and reach a hall with several Spikes. When you get here, you can actual move toward the camera to step outside the building. Bring a the Propeller Block or a Cat Suit, then scramble up to an awning out here. You can then jump to a cloud platform where the stamp awaits.
You could propel to the roof and finish the course early now. Instead, collect all the keys to unlock the warp box and reach a hall with several Spikes. Head to the end, but leap over the warp box here to find one last Bowser gong. Hit it to summon a mystery box. Leap inside to find yourself facing yet another Spike. Take him out before time runs out to earn star.
World 6-4: Deep Jungle Drift
Stamp: Cat Suit Toad
When your reach the first island, leave by the upper path. You'll probably need a Cat Suit to get there. Swing across the poison via the trapeze to land on a blue platform where the stamp sits.
If you take the lower path, you'll find this inside an isolated clear pipe. Just make sure the raft has moved far enough ahead that the pipe won't spit you out right into the poison!
Just past the checkpoint is a hollow green star. You'll need a Cat Suit to scramble to the upper level to reach it and spawn eight green coins. Grab them before time runs out to earn a star.
The level's last star is actually a long the wall before the warp box. Use a Cat Suit to scramble up to it.