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

I picked up part-time shifts as a hotel front-desk agent and I am confused about how to report them. I have read a lot of threads here and could not find my exact situation. The form asks for gross earnings, which I do not see until later. Did anyone get an overpayment notice from a mistake?

The claimant handbook has worked examples for part-time pay. Mine was clearer than the phone line.