How do you report hours that vary week to week? after 16 years as a procurement assistant

I picked up part-time shifts as a procurement assistant and I am confused about how to report them. I have read a lot of threads here and could not find my exact situation. The form asks for gross earnings, which I do not see until later. Did anyone get an overpayment notice from a mistake?

Report honestly and keep your stubs. An honest small error is much easier to fix than a missing report.