How do you report hours that vary week to week? after 11 years as a line cook

I picked up part-time shifts as a line cook and I am confused about how to report them. I am in Birmingham, in case local advice matters. Sometimes my paycheck is late by a week. 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.