When Mario has collected 10 stars, a bright light in the castle main room will appear. Stand in the light, with Mario facing the stairs and press and look up. Mario will then enter the level.
Finding the Switch
The level is basically composed of four pillars and a large tower in the center. Fly to the center tower, and the red Wing Cap Switch is on there. Press it, and now all the red blocks will become solid. Now you can become Wing Mario, and take to the skies by triple-jumping or by being shot out of a cannon!
The eight red coins are placed in pairs in the air around the towers. You will need to use the wing cap to fly round and collect them. One pair that is directly in front of you is so high up in the air that you cannot land before collecting it, otherwise you will not be able to jump high enough from the tower to reach it. The recommended route is to head directly forwards for the first and highest pair of red coins, then turn left and follow the coin trail spiralling downwards to get the next 3 pairs. You should jump on the red switch before getting the star to unlock the wing cap, which is required to get other stars in the game. Jump off the tower and press to exit the map and retry if you don't get all of them in one run. You will be returned to the castle with no lives lost, so do not worry.