How do you report hours that vary week to week?

I picked up part-time shifts as a teaching assistant and I am confused about how to report them. I do not usually post, so bear with me. 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.

Keep everything in writing and save the confirmation numbers. It saved me later.