Got an overpayment notice, what now?, electrician's helper here

The agency says I reported earnings incorrectly, but I think I followed the instructions. Any experience at all would help. I did not notice any errors at the time. 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.

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.