Courtyard stars!

I have no idea which secret stars you have so I'll try to list them all as best as I can.

Three secret stars are given to you by Toads in the castle. They are the ones near Tick Tick Clock, Hazy Maze Cave, and Tall Tall Mountain.

Two stars are in the Princess' secret slide. Finish it to get one, then finish it again under 21 seconds for the second one.

There is a star near the painting of Jolly Roger Bay. Just go to the hole in the wall on the left and collect the re coins (the other hole contains a 1-Up mushroom)

You probably have the one in the painting of the beach area in the room where the minigames are unlocked by gathering the 5 silver stars.

There are two stars in the painting of Goomboss. Collect the Red coins for one and hit a switch star for another (requires Mario).

You can get one by capturing the glowing rabbits, located in various locations (I'm not sure, but I don't think you can capture them all with one character). After catching them, you can then go into the unmarked room next to Wario's room.

There are two in the first Bowser stage. Again, gather the Red Coins and the Switch Star.

There is one after you collect the red coins from the Boos in the Courtyard

There is another in the courtyard under the bricks leading to a hole.

There are two in Luigi's painting. One is a red coin star. The other requires Luigi. Go to the picture of Boo crying as Luigi while he is invisible. He will go through it and you can collect the star.

There are two behind the waterfall (can access through Hazy Maze Cave), a red coin star, and a switch star.

The second Bowser stage also has two stars (again, red coin and switch)

When you drain the moat, use Wario to break the black brick. Collect the red coins for a star. The other star requires Mario. Get a power flower to go into balloon mode and go straight south from the end of the ramp slide. There is a star on this small area as well.

In the Rainbow extra stage (the one with clouds you can go on) there are two stars. One is a red coin star and one is inside the black brick (requires Wario)

There is another star in the mirror room (where the Wario Painting is). Use Luigi to go through the mirror and then go through the door on the other side to get a star.

There are two stars in Wario's painting. One is a red coin star (again). The other is hidden in black bricks. You will need Wario and a Power Flower to get to them (there is wind).

There are two in the last Bowser stage, a red coin star, and a switch star.

Finally, there is a star in the place where the ? Block switch was. Just gather the red coins to get it.

That's as concise as I can be. Hope this helps.

Six stars are not found in courses. Intrestingly, they don't leave ghost stars. Three Toads will give you stars. The one in HMC requires 20 stars, WDW and TTM 40, and TTC is 60. In the place where you found Wario, turn invisble and go through the door. After beating Bowser at least once, catch eight white rabbits. You already have the courtyard star. You punched them, not knowing you collected the coins or the star. You're missing a diffeerent star. You can find guides on Google.