Reporting part-time earnings while on benefits - 13 months and counting

I picked up part-time shifts as a customer-service rep 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?

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