How?

You go back 2 one of the yellow pipes at the begining. Use Drill Bros. Then go under the gate, and find the thing to make you go up into the sky. Then you will see a blue shell looking block. Hit it by jump, play a minigame, and go back to the lever place. Use your new skill to hit the levers.

Basically, you need to find the Koopa gang, located in the sky directly to the east of the town. they will teach you how to wear a shell, and shoot mario across short gaps with the shell. Then go back to the switch area, and use the shell whack move to turn them. But only hit the horizontal part, not the center that sticks up.

What you do is you get the shell at the top of the stairs but DO NOT jump down them. Just walk down them. Then have luigi whack you to the sides of levers to make the briges change and hit all 3 ! blocks to make the elevator open and go into it.

You go back to where the purple pipe is go into the closesed yellow pipe, up the steps, do the twirl move on the air comming out of the hole keep doing the move on all of the pieces of air until you reach the point where you can't go any further hit the blue shell block and three koopas will come and teach you how to use the shell.
Then head back to the puzzeled toad hid the shell block and hit all of the leavers until the mushroom door opens and you've passed the area and you will meet Fawful.

If you have the action replay card you can make Mario and luigi moon jump to hit the switches