How do you report hours that vary week to week? after 18 years as a welding apprentice

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I worked as a welding apprentice before this. 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.