Once you reach a grand total of 10 Power Stars, head to the main lobby and you should see a light shining down near the base of the stairs. Stand on the sun icon that’s on the ground, push the right analog stick forward to go into look mode, then look straight up where the sun is shining down to be transported to a secret level. This is the Wing Cap Course, where you will unlock the Wing Cap for any normal course that has the red blocks. There will be a large red switch located on the tower below you, but there is also a Power Star that can be gotten from collecting all eight Red Coins.
The Red Coins are located in a spiral pattern along the outer portion of the map and you will need to get them upon first coming into the secret area. The reason for this is because you simply cannot get enough height from taking off from the tower below, where the red switch is located. The Red Coins will be in pairs, but it can be pretty easy to miss one of the two in each pair, so take things a little slowly, but not too slowly that you lose altitude. You can nosedive for a second or two, then pull back on the analog stick to get a little extra boost back into the air. It will take a little time to get used to things, but you’ll get the hang of it and get the star in no time.