How do you report hours that vary week to week? - 12 months and counting

My hours change every week at my new part-time job and I am afraid of reporting wrongly. This is my first time dealing with any of 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?

Thanks for writing this out. I went through something close to it 12 months ago, and it settled down once I kept a simple log of every call and form.

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