Don't worry, it's pretty simple. Once you get ten or eleven stars, there should be sunlight streaming down to the big sun on the floor of the castle lobby. Switch the camera angle so you're looking from behind Mario's head, whatever that camera angle's called. Then look up into the sunlight, and you should see the sun and two clouds painted on the ceiling, and you'll be transported to the Wing Cap place. Fly down to the button and jump on it, causing the wing caps to appear. You can enter that level when ever you want after that, like to get the red coins.
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