How do you report hours that vary week to week?, IT help-desk technician here

I work some days through a staffing agency while still certifying. This is my first time dealing with any of this. The form asks for gross earnings, which I do not see until later. Did anyone get an overpayment notice from a mistake?

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

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.