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

I picked up part-time shifts as an IT help-desk technician and I am confused about how to report them. I have read a lot of threads here and could not find my exact situation. Pay posts on a different day from when I work. 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 settled down once I kept a simple log of every call and form.