How do you report hours that vary week to week?

I picked up part-time shifts as a paralegal and I am confused about how to report them. I would rather hear the honest version than the polite one. 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 3 months ago, and it got easier once I kept a simple log of every call and form.