This series of glitches requires you to know how to do a backwards long jump. Do a long jump (Run, Z then A really fast) and point the joystick behind Mario. I do not own these pictures. I took moments of a YouTube video by "Swordless Link."
Step 1: Bowser in the Dark World
Make a brand new account. Then, you will see Princess Peach writing a letter to Mario and introductions to the game. Go inside the castle and head over to the staircase with the door that has a "3" star on it and face the wall. Then, perform a long jump but after you are in mid air, point the control stick behind you and press A as rapidly as possible. It takes a lot of practice, but Mario should be doing a whole bunch of backwards long jumps extremely fast and he should now be backwards long jumping just to the right of a short pillar with two coins on it. If you keep up the backwards long jumping he should end up in the 1st Bowser course. Complete the course and defeat Bowser and go to the basement.
Step 2: Bowser in the Fire Sea
After you've entered the basement, you should see a small staircase leading down to the rest of the basement. Go to the top of the staircase. Make sure your back is facing a star door and do exactly what you did in Step 1. If done correctly, it should produce the same effect although you should now be backwards long jumping just to the right of a tall pillar holding up the ceiling right next to the star door. If you continue backwards long jumping then Mario will go through the star door, bypass the entrance to Dire Dire Docks and enter the second Bowser course. Complete the level and defeat Bowser and head up to the second floor.
Step 3: Bowser in the Sky
After you go up a spiral staircase, you will be in a room with several paintings. There will be a rather larger staircase to your right. Go up it a little, then turn around and do a long jump. Then, point the control stick backwards and tap A as fast as possible and backwards long jump up the stairs. It may take a few tries, but eventually Mario should zoom up the stairs with incredible speed, shooting you through the star door at the top of the stairs and into another room. Go up the huge steps in the back of the room. There will be another star door at the top above the one you just came out of. Go up to it and it will say that you need to have 70 stars before you can get up to the next Bowser level. The star door will open and there will be another staircase that you can't climb unless you have 70 stars. Go up it a little and do what you did with the previous staircase. If done correctly, once again Mario should shoot up the stairs allowing you to complete the 3rd Bowser course. Defeat Bowser once more and collect the star he gives you and you beat the game with 0 stars.