How do you report hours that vary week to week? - what worked for you?

I picked up part-time shifts as a software tester 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?

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