How do you report hours that vary week to week? (update)

I picked up part-time shifts as an accountant and I am confused about how to report them. There are 9 people counting on me, so I am trying to be careful. Sometimes my paycheck is late by a week. Did anyone get an overpayment notice from a mistake?

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