Reporting part-time earnings while on benefits - any advice?

I work some days through a staffing agency while still certifying. Any experience at all would help. 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.

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.