Got a denial letter for unemployment, what now? - any advice?

I got a denial letter last week saying I left my job voluntarily, but I was let go as a mechanic. I am in Denver, in case local advice matters. My employer's version of events does not match mine, and I have emails that show it. 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.