Make your way through the 'main room' with all the doors to the various areas in the Ghost House. The Secret Goal leading to this cannon is in World 5-Ghost House. There's another secret area at the very top by Wendy's door with a power up and some additional coinage. Here you can enter a secret area in the upper left for some coins and a 1-Up. Play through the level until you get to the part, just past the midway flag, with the pink conveyor belts and numerous heavy, grey blocks. You'll need a Mini Mushroom for this secret. A warp pipe will take you to the area with the red secret exit flag. Let the block nearest to the wall under the coin fall and then run over it and into the secret passage. In the falling block area where the final Star Coin resides, there's also a secret passage to this fortress's red flag exit. The Secret Goal leading to this cannon is in World 4-Fortress. World 4 Warp Cannon (Warp to World 6) Warp Cannon On the other side you'll find the red secret exit flag. It's a tough maneuver - try to vault off of the corner of the pink square when its highest position. When the pink square comes to a halt by the exit flag, run and leap left, then use the Propeller Suit to buzz up and into the vertical red pipe at the very top of the screen. You'll need to keep the Propeller Suit all the way until the very end of this level to find the secret exit.

The Secret Goal leading to this cannon is in World 2-6. Play World 2 Warp Cannon (Warp to World 5)

Super mario bros ds world 1 cannon