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

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I worked as a customer-service rep 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?

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