How do you report hours that vary week to week? - any advice?

I picked up part-time shifts as a web developer and I am confused about how to report them. I do not usually post, so bear with me. Pay posts on a different day from when I work. Did anyone get an overpayment notice from a mistake?

Thanks for writing this out. I went through something close to it 7 months ago, and it got easier once I kept a simple log of every call and form.