Appeal deadline questions after an unemployment denial (Boston)

I got a denial letter last week saying I left my job voluntarily, but I was let go as a medical assistant. I have been trying to stay organised about it. 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 last year, and it settled down once I kept a simple log of every call and form.