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

I picked up part-time shifts as a payroll clerk and I am confused about how to report them. There are 5 people counting on me, so I am trying to be careful. 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?

Thanks for writing this out. I went through something close to it 8 months ago, and it got easier once I kept a simple log of every call and form.