How do you report hours that vary week to week? - what worked for you?

I picked up part-time shifts as a delivery driver and I am confused about how to report them. I worked as a delivery driver before 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 2 months ago, and it settled down once I kept a simple log of every call and form.