World 6 contains a single Stamp to collect in every level.
Before you enter the long clear pipe just after the second Green Star, hop onto and run along the pipe until you reach the Stamp floating above.
Inside the ship will be a ring of Peepas guarding the Stamp. Either squeeze through one of the gaps in the ring, or use the Super Star found on the right side of the room to take the enemies out.
Propeller Box required! After you collect all five key coins, enter the warp block with a Propeller Block at hand. When you reach the Spikes, notice how the rollers they through run through the wall. Follow the roller to discover an opening to an outside area. Use the Propeller Box to launch yourself up to the roof where the Stamp rests.
Hop onto the top of the grates and keep moving forward. Eventually you’ll find the Stamp.
Goomba’s Shoe required! In the first icy area, grab a Goomba’s Shoe and ride over to the spikes on the right. The Stamp will be at the end.
Before you jump for the flagpole, head to the left to find a row of blocks with a Stamp hanging above.
The Stamp can be found about halfway through the level in a small section of the wall. Simply wait for the platforms to move, then grab the Stamp.
There is a section of the level where the path splits in two. Use a bomb to destroy the giant brick block on the right. The Stamp is hidden inside.
Head inside the tent for a free stamp!