How do you report hours that vary week to week? - 3 months and counting

I picked up part-time shifts as an event coordinator and I am confused about how to report them. Any experience at all would help. 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.