How do you report hours that vary week to week? - first time posting

I picked up part-time shifts as a data-entry clerk and I am confused about how to report them. I do not usually post, so bear with me. The form asks for gross earnings, which I do not see until later. Did anyone get an overpayment notice from a mistake?

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.