How do you report hours that vary week to week? - 20 weeks in

I picked up part-time shifts as a teaching assistant and I am confused about how to report them. This is my first time dealing with any of 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 9 months ago, and it got easier once I kept a simple log of every call and form.