In the first part of the level, you can see yourself, but the shadows are very prominent. Get a Super Bell and pick up a Potted Piranha Plant, which will eat anything in your way, and pick up things for you, such as Green Stars. You'll encounter Fire Piranhas and Fuzzies. Have the Potted Piranha Plant eat them. Go to the right and you'll see the shadow of a Green Star. Walk towards the front wall and you'll find the area extends far forward. Keep going to reach the Green Star. If you stand on the platforms near the front wall, you may find something. Soon, you'll find a midpoint and a door.
The door leads to the other side of the gap, but now all you can see is shadow, making the area 2D. You'll get another Potted Piranha Plant to use on the Fuzzies. Ahead is a Giant Piranha Plant. Stomp it, then drop down and use your claws to reach the stamp below. There are crates that look like walls are blocking your way. At the end of the area is a door.
The door will bring you out of the shadows. Pick up another Potted Piranha Plant. There are pink enemies covering part of the wall. Tap them on the touch screen or have the Piranha Plant eat them. Behind them is a Mystery Box. Hit the P-Switch on the stairs to reveal that the second Green Star is hidden under the stairs. Drop down, then go left to get it. Pick up the Potted Piranha Plant again and have it eat the Fuzzies. Go past the next door you see. You'll find Captain Toad cowering near the shadow of Bowser. Walk towards the front wall to reveal that it is really animatronic cardboard. Use your claws, fire balls, or just knock it over to destroy it. Captain Toad will give you the third Green Star. Go through the door to reach the flagpole.
The stamp is of the Potted Piranha Plant.