When you go upstairs and have one Star, you can go through a door with a number one on it. When you go in, go through one of the pictures of Peach. One of them contains a level with a secret Star.
When you go into Luigi's picture frame (located in the ghost place), get Luigi's cap. Find a "?" box after killing everything in there. When you use a Power Flower and you are invisible, in the final room where you enter the hole to face King Boo, there will be a Star on the map. Because you can go through walls, go through the ghost picture and there will be a Star.
As Luigi, from the main hallway go into the door at the top of the first staircase. Go up the stairs and into the door at the top of this staircase. Then, go into the door with red carpet around it. There will be a room with a huge mirror in it. Get a flower on one of the pedestals and go through the mirror. Enter the door in the reflection. You will be in a room that is all yellow, with a lone Star.
Go to the rec room where you play minigames. Jump into the painting in that room. Collect the five Blue Stars to get the real Star to appear. This is also a level that appears when you play in multiplayer mode.
Go to the second floor with Luigi. There is a 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. Before he finishes, he also gives you a Star.
Go into the basement with Wario. Go to the entrance to Hazy Maze Cave but do not jump in. Go all the way around and down the steps. You should see a Toad in the left hand corner. Speak to him for a Star.
There are secret Stars in the hole in wall across from rainbow place. You can get one by collecting red coins and one in a black brick. First, fly to the place with Wario's cap then use the canon. Aim it to the tree and put it slightly higher. Break the black brick and a Star will appear.
In all the levels leading up to a Bowser fight (the levels that bring you to a transport pipe), there are a possible of two secret Stars. One is to find all eight red coins. The other is to find a switch Star by stepping on a Star button.
Finish the secret level in the character selection room in less than 21 seconds to get another Star.
On every course in this game, you are able to get a Star if you collect 100 Coins.