Wage reporting when hours change weekly after 18 years as a teaching assistant

I picked up part-time shifts as a teaching assistant and I am confused about how to report them. Pay posts on a different day from when I work. 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.

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.