Sources are what make history - without them we wouldn't know what actually happened in the past. So, how does this work in an exam setting with a source based question? Relax, first and foremost, you're likely prepared for this and all of your hard work is going to pay off. The first thing we have to do is establish the provenance of the source - where did it come from? Who wrote it? Who was the intended audience? Is it contemporary or a commentary made from a later date? When we've firmly established the provenance we need to expand off this. Do you know of any scholarly debates surrounding the source itself? If so great - mention them as it shows you have a wider awareness in regards to the historiography. Finally, we need to get into the bulk of the source itself. It's all well and good discussing where it came from and what historians think about it, but if we don't address its content then we won't push forward into the higher grade boundaries. We need to take this evidence and link it back to the question - the best system I found for this was PEAL: Point, Evidence, Analysis, Link. It's always important to link any argument you've made back to the original question.
To summarise, we have to establish the historical provenance, comment on historians opinions and discuss the content of the source itself whilst linking it back to the question. If you remember all of this and provide a detailed and confident answer then you should do well with any source question.