Go to the tower in World 1 with the Blue Shell power-up. Go to the first pipe that shoots you up into the air. On your way back down, go in the gold door. There will be a lot of randomly moving platforms. On the right, there is a gap that always stays the same size. Go through it and you will see three blocks. Use the Blue Shell ability to break them all, then break the last three. Go to the right and finish the level to reveal the World 1 Warp Cannon, which will shoot you to World 5.