How do you report hours that vary week to week? - 8 months and counting

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I have been trying to stay organised about it. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 8 months ago, and it settled down once I kept a simple log of every call and form.