In this circus-themed level, there are a whole bunch of blue panels. Turn them all yellow to progress!
Before the checkpoint
The stage begins with a simple arrangement of blue switches. If you step on one, it turns yellow; step again and it goes back to blue. Once all of them are lit up, the back wall is removed and you can progress.
The next room has some bees flying around. Since they home in on you, jump straight up before they collide and they'll all congregate beneath your boots. Use the springboards to climb the switches and continue on to a room with a whole bunch of bees. Before heading into the warp box, wall-kick up the sides of the room to reach the first green star. Once that's done, the warp box will take you to the checkpoint.
After the checkpoint
The maze of switches after the checkpoint can get a bit tricky, but as long as avoiding the enemies is your top priority you should be fine. Go through the heart wall for a climb and another red ring challenge that can net you a fire flower. Halfway up is a challenge box with the second green star.
At the top is the last wall you need to pass, and it's guarded by a magikoopa. Be careful when hitting the switches, and make sure to nab the last green star in the middle.
After going through the cannon pipe, the end of the level is in sight. If you want the stamp, solve the switches and wall-kick up the backdrop. Otherwise, you can head straight to the flagpole.