Wage reporting when hours change weekly after 15 years as a mechanic

My hours change every week at my new part-time job and I am afraid of reporting wrongly. There are 15 people counting on me, so I am trying to be careful. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

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