To start a bonus Play game, select New Game from the main Menu, chosing a difficulty equal or easier to one you have already completed. When it asks If you want to play with all your upgrades, select Yes and it will then let you choose from the costumes you have unlocked, or choose no costume.
A cut scene plays out and introduces you to the ferryman, Charon. Know this: The fight with Charon is impossible to win, so don't sweat it too much if you get hit. If you wanna fight him anyway, do so by jumping over his head and attacking with Rampage of the Furies over and over.