How to write "I quit" politely?

So I work in fast food and have really just been looking for an excuse to leave this place and never look back. I'm having a major dilemma right now because I was given short notice that I would be busy this Friday, but I only found out the exact day I had gotten my schedule. I tried texting the general manager and the manager who works that shift I need to call out on and both of them claim that I needed to be more prepared even though I told them that I didn't know until the day I had gotten my schedule. I've already applied to Several jobs and am getting someone I know to put in a good word for me somewhere else, I'm really set on quitting through text because I don't have any other days that I work before Friday, how do I word it?