Reporting part-time earnings while on benefits after 12 years as a mechanic

I picked up part-time shifts as a mechanic and I am confused about how to report them. There are 12 people counting on me, so I am trying to be careful. 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.