Got an overpayment notice, what now? after 12 years as an event coordinator

The agency says I reported earnings incorrectly, but I think I followed the instructions. I worked as an event coordinator before this. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.

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