Wage reporting when hours change weekly - any advice?

I picked up part-time shifts as a bookkeeper and I am confused about how to report them. I have been trying to stay organised about it. 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 got easier once I kept a simple log of every call and form.