You need Luigi or a Luigi cap to do this and you also need to have unlocked the blocks. Go to the second floor of Boo's Mansion (wall bounce, back flip or double jump onto it from the top right room in the first floor). Hit the block as Luigi or wearing a Luigi cap and pick up the power flower. While invisible run through the blue picture of a Boo on the second floor. You'll be in a new room with a giant eye in it. Run in circles around the eye a few times to defeat it and obtain the star.
If you mean the one next to Wario's room which is locked, it's just a secret Power Star.
get all the star coins.
yes
One of them is in the consecutive rooms where the star coin doors are and the level editor paintings. There is a secret opening in the room with the level editor paintings. Go through it, and there should be a star coin.
you defeat all of the boos (ghosts) and the star will appear
There is a shop by the enterence of the Mario Stadium. There is also a secret shop at the Daisy Cruiser at night, but you have to unlock it.
The Mush-Room. It's a big room with a lot of 1-UPs in it.
look everywhere! {they might even be in the walls or secret exits.}
Get all the star coins.
Find the secret exit in Star World 5. To do this, you will need to have found all Switch palaces.
There is no star mushroom in Super Mario Galaxy 2.
you need a certain amount of stars