Appeal deadline questions after an unemployment denial

I got a denial letter last week saying I left my job voluntarily, but I was let go as a software tester. 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?

I printed my emails in order and wrote a one-page timeline. The hearing officer kept going back to it.

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.