How do you report hours that vary week to week?, IT help-desk technician here

I picked up part-time shifts as an IT help-desk technician and I am confused about how to report them. I worked as an IT help-desk technician before this. 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?

Report honestly and keep your stubs. An honest small error is much easier to fix than a missing report.

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