How do you report hours that vary week to week? - what worked for you?

I picked up part-time shifts as a welding apprentice and I am confused about how to report them. It has been a long 10 weeks. 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?

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