How do you report hours that vary week to week? after 4 years as a nurse's aide

My hours change every week at my new part-time job and I am afraid of reporting wrongly. There are 4 people counting on me, so I am trying to be careful. 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?

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.