Reporting part-time earnings while on benefits after 10 years as an IT help-desk technician

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I have been trying to stay organised about it. Sometimes my paycheck is late by a week. Do you report when you earned it or when you were paid?

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.