How do you report hours that vary week to week? after 12 years as a project coordinator

I picked up part-time shifts as a project coordinator and I am confused about how to report them. It has been weighing on me all week. The form asks for gross earnings, which I do not see until later. Do you report when you earned it or when you were paid?

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