Appeal deadline questions after an unemployment denial after 11 years as a dental hygienist

I got a denial letter last week saying I left my job voluntarily, but I was let go as a dental hygienist. I am in Chicago, in case local advice matters. The letter lists a deadline and a hearing process, but it is hard to follow. If you have appealed before, what helped you prepare?

Thanks for writing this out. I went through something close to it 2 months ago, and it settled down once I kept a simple log of every call and form.

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