Reporting part-time earnings while on benefits - what worked for you?

I work some days through a staffing agency while still certifying. There are 18 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.