Reporting part-time earnings while on benefits

I picked up part-time shifts as a software tester and I am confused about how to report them. There are 3 people counting on me, so I am trying to be careful. Pay posts on a different day from when I work. 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.