How do you report hours that vary week to week? after 7 years as a teaching assistant

My hours change every week at my new part-time job and I am afraid of reporting wrongly. There are 7 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.