Got a denial letter for unemployment, what now? - 13 weeks in

I got a denial letter last week saying I left my job voluntarily, but I was let go as a customer-service rep. I worked as a customer-service rep before this. 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 3 months ago, and it got easier once I kept a simple log of every call and form.