Reporting part-time earnings while on benefits - 15 weeks in

My hours change every week at my new part-time job and I am afraid of reporting wrongly. Any experience at all would help. 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 4 months ago, and it got easier once I kept a simple log of every call and form.