How do you get the grandmaster galaxy in super Mario galaxy 2?

what you have to do is complete super Mario galaxy with 120 stars, then go to rosalina and speak to her. She will ask you do you want to go to the center of the universe. click yes and go there. Once you have beat bowser, after all of the credits, a message will appear saying you can now play as luigi. then, go to the main menu and press a and b simultaniousy. then go on to the file you've got 120 stars on. it will have Mario AND luigi on there. Just click on who you want to play as. If you want grand finale galaxy, complete it with 120 luigi stars and go back to the center of the universe. click yes and go on it and complete it. Once again, when you beat bowser and the credits have gone, there will be a message saying you can go on grand finale galaxy. Go to the main menu and go to the file where you have got both Mario and luigi playable and go on it. Once you're on it, go to the planet of trials and grand finale galaxy will be on the back of it. ( It will have that on both Mario and luigi).