I wing it. You know that sensation of DM scrambling you feel when your party goes off the rails and is somewhere doing something you never expected?
I just DM a short session in that way.
I tell them what they are, race/class wise, then ask them to think about what kind of person this is, and to act like that person. I don’t use sheets, just scrap paper and some dice, and the plot that I take them through is probably going to very closely resemble an episode of a show or the intro to a movie I’ve watched recently. I follow the actual rules insofar as they are convenient, but what I’m really doing is just trying to show them what role playing feels like.