Verified answer

you stand at the start of the room.

start dashing (hold down "y" and hold down "right").

after a few seconds mario will automatically shell dash.

from that point, you only need to hold down Y (or X).

once you let go of Y, you stop shell dashing.

while shell dashing, you can jump as much as you want, but can only change direction by bumping into a wall or the other solid objects.

Nintendo Ds Shell

first put on the shell then press the Y button to dash and then you will automatically start spinning as long as there is a straight path

Start running (hold --> and 'Y') and Mario jumps into the shell.