How do you report hours that vary week to week? (update)

I picked up part-time shifts as a medical assistant and I am confused about how to report them. Thank you in advance. 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 4 months ago, and it got easier once I kept a simple log of every call and form.