Wage reporting when hours change weekly - any advice?

I work some days through a staffing agency while still certifying. I have read a lot of threads here and could not find my exact situation. 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?

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.

Thanks for writing this out. I went through something close to it 5 months ago, and it settled down once I kept a simple log of every call and form.