How do you report hours that vary week to week? - what worked for you?

I picked up part-time shifts as a pharmacy technician and I am confused about how to report them. I do not usually post, so bear with me. The form asks for gross earnings, which I do not see until later. Do you report when you earned it or when you were paid?

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.

Thanks for writing this out. I went through something close to it 5 months ago, and it settled down once I kept a simple log of every call and form.