How do you report hours that vary week to week?

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I have read a lot of threads here and could not find my exact situation. 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 last year, and it got easier once I kept a simple log of every call and form.