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

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. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 2 months ago, and it got easier once I kept a simple log of every call and form.