Appeal deadline questions after an unemployment denial - any advice?

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

Thanks for writing this out. I went through something close to it 6 months ago, and it got easier once I kept a simple log of every call and form.