How do you report hours that vary week to week? - 13 months and counting

I work some days through a staffing agency while still certifying. I have been trying to stay organised about it. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 13 months ago, and it got easier once I kept a simple log of every call and form.