Wage reporting when hours change weekly - 10 months and counting

I picked up part-time shifts as an event coordinator and I am confused about how to report them. 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 10 months ago, and it settled down once I kept a simple log of every call and form.