Denied unemployment, how do I appeal? after 12 years as a sales associate

I got a denial letter last week saying I left my job voluntarily, but I was let go as a sales associate. I am in San Diego, 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?

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

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