[33girl]

adpiucf has the right idea, although I would go into a bit more detail about WHY exactly you're leaving, because in some cases the EO or the chapter will not accept a self-termination that seems to be for no real reason.

You don't have to be rude - just state what happened. If it's something that's causing you to terminate, the EO will want to know in case it's a problem that keeps coming up over and over again in the chapter that they can hopefully fix. Example - if you're the 3rd girl in a year to quit because there were too many mandatory events and it was interfering with your life and grades, then the EO will probably want to talk to the chapter about overprogramming.