How do you report hours that vary week to week?

I work some days through a staffing agency while still certifying. It has been a long 4 weeks. 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 7 months ago, and it settled down once I kept a simple log of every call and form.