How do I answer a 'how useful' source question?

BEFORE reading the sources, read the question and jot down key points you remember. Then read the source and notice areas/important people that the source does mention AND what it does not. Then answer the question.
Q: How useful is Source C as evidence of the Massacre of Glencoe? (Maximum of 6 marks)
Maximum of 4 marks for evaluative comments relating to author (WHO wrote it); purpose (WHY they wrote it); type of source (HOW they wrote it) and the timing (WHEN they wrote it)
For example: Source A was written the day before the Massacre, which took place on the 13th of February 1692. This makes it useful because it was written immediately before the massacre (1 mark for origin: WHEN). It was written to Captain Robert Campbell who was in charge of the soldiers who carried out the massacre. It is useful therefore because he was at the centre of the decision making to carry out the massacre (1 mark for origin: WHO).
Up to 2 marks for interpretation of content (WHAT the source says + WHY that's important)
For example: The source is useful because Duncanson tells Campbell that if he does not follow the orders than he will lose his job (WHAT). This is useful because it shows that Duncanson knew that the order to 'kill all under 70' would be unpopular after the troops had spent two weeks living with the clan and thus needed to provide incentive/a threat for them to be carry it out (WHY important) (WHAT + WHY = 1 mark)
There are also 2 marks available for important things that the source misses out which follow the same structure (WHAT the source does NOT SAY + why its important)
For example: The source is limited because it was written before the massacre occurred and therefore doesn't provide any details of what actually happened (WHAT). For example that the massacre was botched because most of the clan fled into the hills after the escape routes were left open (WHY IMPORTANT).