How do you report hours that vary week to week?

I picked up part-time shifts as a welding apprentice and I am confused about how to report them. I would rather hear the honest version than the polite one. 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?

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