Instructions
Play through the level in Toad Town until you learn the "turtle shell" skill. A tutorial will show you how to use it. Go down into the Toad Town caves.
Go left, then up the ramp and left into the next area. Go up and to the right along the path. Jump into the shell block to change into Turtle Mario.
Hit Mario with Luigi's hammer to send Mario to a lever in the water. Pull the lever to make a bridge appear. Walk across the bridge and jump into the "!" block to make another turtle shell.
Pick up a turtle shell and cross the next bridge. Turn right and you'll see three switches.
Throw a turtle shell at the upper right switch so it's pointing vertically. Throw a turtle shell at both the switch on the lower right and the switch on the upper left so they're pointing horizontally. This will cause another bridge to appear that you can cross to get into the next room.