How do you report hours that vary week to week? after 5 years as a project coordinator

I picked up part-time shifts as a project coordinator and I am confused about how to report them. I have read a lot of threads here and could not find my exact situation. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 3 months ago, and it settled down once I kept a simple log of every call and form.