How do you report hours that vary week to week? after 3 years as a welding apprentice

I picked up part-time shifts as a welding apprentice and I am confused about how to report them. I am in Portland, in case local advice matters. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.

Thanks for writing this out. I went through something close to it 2 months ago, and it settled down once I kept a simple log of every call and form.