How do you report hours that vary week to week? - 10 months and counting

I picked up part-time shifts as a web developer and I am confused about how to report them. I have been trying to stay organised about it. Sometimes my paycheck is late by a week. Did anyone get an overpayment notice from a mistake?

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.