Got a denial letter for unemployment, what now? - any advice?

My claim was denied after 12 weeks of waiting, and the letter says I have a short window to appeal. I do not usually post, so bear with me. 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.