Got a denial letter for unemployment, what now? (update)

My claim was denied after 12 weeks of waiting, and the letter says I have a short window to appeal. I have read a lot of threads here and could not find my exact situation. 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.