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. There are 8 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?

Keep everything in writing and save the confirmation numbers. It saved me later.

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