Got a denial letter for unemployment, what now? (update)

I got a denial letter last week saying I left my job voluntarily, but I was let go as a security officer. Any experience at all would help. My employer's version of events does not match mine, and I have emails that show it. If you have appealed before, what helped you prepare?

File the appeal before the deadline, even if you are unsure. You can gather documents afterwards. That is the one part I would not wait on.