How do you report hours that vary week to week? - first time posting

I picked up part-time shifts as a security officer and I am confused about how to report them. I do not usually post, so bear with me. The form asks for gross earnings, which I do not see until later.

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

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