This stage is Bowser’s Highway Showdown (1-Castle). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars.
Game Play Walkthrough Video
Green Star 1
Collect the 8 green coins after triggering the big green star ring.
Green Star 2
Use the cat’s paw to turn on the wheel. While the platforms are raised, climb up and jump to the right to collect your second green star.
Green Star 3
The last green star is located on top of stack of Goombas. You can either kick a soccer ball to destroy most of the stack or kill them one by one.
The stamp is hidden near the Green Star 2. You will have to kick the soccer ball to break the stone wall to reveal the stamp.
Bowser Fighting Tips
The first boss of the game is pretty easy, all you have to do is to kick the soccer ball towards Bowser. You should try to maintain your cat suit during this boss fight as it is the most effective way.