This stage is Bullet Bill Base (6-6). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret 1 UP.
Game Play Walkthrough Video
Green Star 1
You have two ways to get this Green Star, either with the Cat Power-Up to climb walls or just going with the cloud platforms. The green star is located at the beginning to the left of the course at the highest point.
Green Star 2
The second green star is at a raised platform near the Checkpoint Flag. Enter the purple mystery box and start the mini course.
Green Star 3
The third green star is found near a row of conveyor belts. You will need the Cat Power-Up to grab this green star.
The stamp is found at the end goal pole of the stage, head left instead of going for the goal for the stage stamp.
Other Notable Secrets
Before the third green star you will see a sign pointing right. There is a platform with nothing right on top. Just parallel to the platform there is a secret box holding a 1UP.