Go to stage one (Bob-omb Battlefield), star 3. Run up to the most northeast portion of the level (where Koopa the Quick is after you beat him) and find the corner of the level where the two grassy hills' walls meet. Line yourself up with the corner and do a long jump up the corner, but try to make it only a little over halfway up. Then, jump again, except this time have Mario jump onto his stomach. Slide up the hill towards the sky still lined up with the corner and if you reach the very top, turn a little to the left and you should start sliding in place. Change the angle of your camera and you'll see that you are out of the level, floating in the sky. To get down, just change Mario's direction so he is facing the level.
Submitted by: Mitchell VanRassel