Got a denial letter for unemployment, what now?

My claim was denied after 13 weeks of waiting, and the letter says I have a short window to appeal. I have been trying to stay organised about it. I have not done a hearing before and I am nervous. If you have appealed before, what helped you prepare?

Thanks for writing this out. I went through something close to it 5 months ago, and it got easier once I kept a simple log of every call and form.