Enter the room accessible only with 50 Stars and, while facing the door, go to the left staircase and jump to the second step. Line yourself up with the corner of this step and the next one so that you are at about a 45 degree angle, then charge up a dash. When you are at about a second away from the corner, slide kick (Crouch + Punch) and hold Up/Left to launch up into darkness before falling down. Attempt to angle yourself so you will land in the room with the portal. Everything will be black and you will hear the endless staircase music. Try to find the hole leading to Bowser and jump in. Note: This can be done with any character, but if you’re not Mario you will die once the Rainbow Road credits end. You can also possibly die automatically if something went wrong when at the staircase.