How do you report hours that vary week to week? (update)

My hours change every week at my new part-time job and I am afraid of reporting wrongly. There are 12 people counting on me, so I am trying to be careful. The form asks for gross earnings, which I do not see until later.

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.