How do you report hours that vary week to week? - 18 weeks in

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I worked as a customer-service rep before this. Pay posts on a different day from when I work. Do you report when you earned it or when you were paid?

Thanks for writing this out. I went through something close to it 11 months ago, and it got easier once I kept a simple log of every call and form.