Got an overpayment notice, what now? - any advice?

The agency says I reported earnings incorrectly, but I think I followed the instructions. I have been trying to stay organised about it. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Keep everything in writing and save the confirmation numbers. It saved me later.

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.