That depends on which bosses you want to fight.
Going the hidden way to the left, you'll find a Hulk - like boss, and afterwards, Mother Brain. Going the hidden left way, jump on top of the bullet bills, or else you'll get blown up by some laser beam. Get the bullet bills to shoot the person - thingy. Then, prepare for tetris. DON'T LET THE PILL HIT YOU.
Going the up way, You'll find Mike Tyson, and (eventually, i don't know when) Mecha Birdo. By the way, going the up way, jump on to the falling spike wall - thing. You'll know once you get there.
From here, you're on your own.
Happy faliure!