How do you report hours that vary week to week? - first time posting

I picked up part-time shifts as a copywriter and I am confused about how to report them. 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 3 months ago, and it got easier once I kept a simple log of every call and form.