Go up the stairs with one star to go through a door with a number one on it. When you go in, go through one of the pictures of Peach to find a level with a secret Star.
Go to the rec room where you play minigames and jump into the painting in that room. Collect the five Blue Stars to make the real Star to appear. This is also a level for when you play in multiplayer mode.
Finish the secret level in the character selection room in less than 21 seconds to get an extra Star.
Go into the basement with Wario and head to the entrance to Hazy Maze Cave, but do not jump in. Go all the way around and down the steps to find a Toad in the left hand corner that will give you a Star.
Go to the second floor with Luigi and find the Toad across from the Tall Tall Mountain entrance, located directly behind the steps that go to the third floor. Speak to the Toad and he will tell you about the mirror room and give you a Star.
In all the levels leading up to a Bowser fight (the levels that bring you to a transport pipe), there are always two possible secret Stars. One is by finding all eight red coins. The other is by finding a switch Star by stepping on a Star button.
On every course in this game, you can get a Star if you collect 100 Coins.
When you go into Luigi's picture frame (located in the ghost place), get Luigi's cap and find a "?" box after killing everything in there (since you will need to go very fast). When you use a Power Flower and are invisible, in the final room where you enter the hole to face King Boo, there will be a Star on the map. Since you can go through walls, go through the ghost picture and there will be a Star.
There are secret Stars in the hole in walls across from rainbow place. You can obtain one by collecting red coins and one in a black brick. First, fly to the place with Wario's cap and use the cannon. Aim it to the tree and put it slightly higher, then break the black brick and a Star will appear