Got a denial letter for unemployment, what now? (St. Louis)

I got a denial letter last week saying I left my job voluntarily, but I was let go as a mechanic. I am in St. Louis, 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?

Legal aid offices and some workforce centres help with unemployment hearings at no cost. A call is worth it.

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.