Where do you get star coins

Answer from: Mario Gumball X
Go down to the part where there is water (you should see a P Switch in the middle of the area), and when you see a narrow passage, swim through it(but not as Mega Mario). Once you get out of the passage, you should see two pipes: one is green and one is red. Dodge the Piranha Plant on the green one, then press DOWN on the D-Pad when you land on the red one. You should enter the pipe. Then you'll find yourself in an underwater level. You don't have to collect the coins there, just dodge the Cheep Cheeps and enter the pipe at the end of that area. Then you'll enter a room in the normal level, and you'll see an Question(?) Switch on the ceiling. Don't touch it yet; go to the far left and get the Star Coin. THEN go back to where you saw the ? Switch, and hit it from below. A hill will rise up onto a bridge with two red-shelled Koopa Troopas on it. Run up the hill and jump onto the bridge, and then you'll be back into the normal level.