How do you report hours that vary week to week?

I picked up part-time shifts as a security officer and I am confused about how to report them. 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.

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.