How do you report hours that vary week to week?

I picked up part-time shifts as a machine operator and I am confused about how to report them. 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.

The claimant handbook has worked examples for part-time pay. Mine was clearer than the phone line.