World 8-1: Spiky Spike Bridge
Take a left from the Fire Bros. and carefully cross the spike bridges. Use a Cat Suit to scramble up the pillar at the end of the path to find a star on top.
Immediately after the checkpoint, you'll enter a warp box. On the other side, head left instead of the right way the sign is point. The stamp is at the end of this detour.
Now head the correct way along the spike bridge. When you reach the Fire Bros., take him out and use a Cat Suit to scramble up the pillar. Enter the mystery box here to find a secret area. In here, you'll have ten seconds to scramble over some walls, hit the switch, then grab the star it spawns.
When you reach the platform covered in Parabones, turn left. You'll have to traverse those spiked platforms to reach the end of the level anyway, but there's a star to be had. You'll have to grab it from the falling platform, then jump back to the spiked one before falling. You can do it!
World 8-2: Plessie's Dune Downhill
The first star is hovering above the first part of the level. Bounce off of your enemies' heads to reach it.
After the big drop, continue through the snaking path. When you see a group of Para-Biddybuds, leap over them to snatch the star.
Stamp: Question Mark Block
Just beyond the second star is a pair of Bowser sand sculptures. Pass right between them and keep moving straight - clouds will appear, guiding you to the stamp.
In the next section, you'll pass over some broken pieces of land. When you spot another Bowser sculpture, smash into it. This generates a star on the next sand mound.
World 8-3: Cookie Cogworks
After passing the Giant Piranha Plant, hop onto the next cog. Turn left and hop onto the cog behind you, then ride it up to spot the first star. It is possible to reach this with a jump, but a throw from the Boomerang Suit makes the job much easier.
Stamp: Boomerang Toad
Just after the clear pipe is a line of Spiny Ant Troopers. Grab the Boomerang Flower in their lineup, then take them out. Drop into the hole they were patrolling to grab the stamp.
Right after, you'll encounter two patrols of Spiny Ant Troopers. Use your Boomerang Suit to clear them out again, then drop through the holes they were guarding. Hop into the mystery box down here to enter a challenging secret room. Inside, you'll be atop a spinning wheel. Keep moving forward to stay aground, dodging the blocks and Piranha Plants that roll by. Keep an eye out for the star, then grab it as it flies by.
There's actually a platform behind the flagpole; use a long jump to reach it. You'll find lots of coins, bonus time, and rabbit there. Catch the rabbit to earn a star.
World 8-Train: The Bowser Express
This is atop the very first train car you see. Hit the wide question mark block, then jump in the middle to hit an invisible block. Use this to reach the star.
Shortly after, you'll spot a hollow green star above some pendulous spikes. Hit it to spawn eight green coins; grab them before they disappear to earn a star.
When you reach the tall train car, you'll see the stamp on the roof. Don't worry about getting up there right now. Instead, head through the door into the train car. Ground pound the right question mark block to generate bricks that carry you right up to the stamp.
When you reach the front of the train, drop down to the lower ledge. Take out the Fire Bro. to nab the star behind him.
World 8-4: Footlight Lane
This level is quite tricky thanks to the semi-visible pathway mechanic. When you run past the Parabones, you'll see this floating in the air. Leap to it; a platform will appear underneath you.
As you cross the section with rolling spikes, you'll approach a hollow green star. Hit it to spawn eight green coins on the spikes' path. Grab them before time runs out to earn a star.
Stamp: Bullet Bill
You'll come to a platform targeted by Cat Suit Bullet Bills. Leap to the right to land on an invisible platform. Up the invisible steps here is the stamp.
While navigating the same walkway with the Cat Suit Bullet Bills, look for a narrow path to the left. This leads to a mystery box; hop inside. In here, you'll have ten seconds to reach the star. The walkway is pretty broad, so quickly leap atop the Thwomp to reach your prize.
Stamp: Worried Sprixie
This unlocks after completing World 8-4. Enter from the map to claim the stamp inside.
World 8-5: Deepwater Dungeon
This is found very close to the start of the level. When you ascend the narrow canal, ride the water up and over the spikes to grab it.
When you ascend the very next canal, use a Cat Suit to scramble up the left wall. There's a star in a hidden alcove up here.
After riding Plessie, use your Cat Suit to scramble up the walls you stop near. There's a star on a high ledge up here.
Instead of taking the green warp pipe to the end of the level, use your Cat Suit to scramble up the wall. The cloud up here will shoot you to a secret area. Dash through the course to grab the star at the end.