Hello. Its mastaplaya21 here. To make this work, make sure you have a feather. Then go to the VERY FIRST GHOST HOUSE IN THE GAME. Once you enter the ghost house, fly all the way to the ceiling at the beggining of the level and run across the top of the room. Once you reach the other side of the wall, you will see 4 blocks. Each one contains a 1-up mushroom (make sure you hit each block one at a time so that you can have 4 1-up mushrooms). Then exit through the door at the end of the room. Finish the level, and.... Voila! You have unlocked the Top Secret Level (you can get feathers, mushrooms, fireflowers, and Yoshi there)!!
If you are small mario/luigi when you finish a level carry a block or a trampoline through the end point you get a growth mushroom. If you are big mario/luigi do the same thing for a bonus life mushroom.