sheiny

Best answer as chosen by Asker / Ratings
To get the Red, Wing Cap - first you need to get 10 Stars, then a ray of light will appear at the entrance inside the Castle. Walk under the light and then look up at it to be transported to the Bonus Stage. Collect the 8 Red Coins for a Secret Star and then Ground Pound (A+Z) the Red ! To unlock the Wing Cap in every Level.

To get the Blue, Vanish Cap - once you've beaten Bowser the first time, head to the basement, unlock the door, follow the dark corridor to the right and enter the door. Go right, continue down the corridor and make the first right to a door without a Star - enter it, get in the water and swim under the wall to another room. Here, you'll find 2 pedestals to Ground Pound (A+Z) and drain the water - next, head through the door to be outside the Castle in the moat, make a right and follow the path to a hole. Jump down it, get through the area and find the Blue ! To unlock the Vanish Cap in every Level.

To get the Green, Metal Cap - you'll have to do Hazy Maze believe it or not. Down by that Sea Monster is a door in the wall at the far end - ride the Sea Monster over to the door, go through that door and jump into the Metallic Paining. Collect the 8 Red Coins for a Secret Star and then Ground Pound (A+Z) the Green ! To unlock the Metal Cap in every Level.

beedizzle

The wing cap
In the entrance hall, stand on the yellow circle and look up, and you will be in the wing cap level. Go to the middle tower and jump on the big red button

The metal cap
Somewhere in hazy maze cave there is a metallic painting that looks like the entrance to hazy maze cave. I can't remember exactly where it is but go through the for the metal cap switch

The invisible cap
Once you drain the moat, there will be a hole at the bottom. Go into there and you will be in the invisible cap level