Main Menu -> Singleplayer -> Training -> Practice
Select any two characters and then select the Metropolis stage. A level transition is performed by pressing backwards+A with your opponent in the corner of the stage. The order for the transitions on Metropolis is Right, Right, Left.
So push your opponent all the way to the right and press back+A. Push them to the right and do it again, and finally push them to the left of the stage and do it again.