Wage reporting when hours change weekly - 5 months and counting

I picked up part-time shifts as a teaching assistant and I am confused about how to report them. 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?

The claimant handbook has worked examples for part-time pay. Mine was clearer than the phone line.

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.