How should I respond to questions which ask me to deal with a source?

Your answer needs to deal with a few different aspects of the source while also drawing on your own knowledge to interrogate what you are reading.
Firstly, you need to show that you understand what the source is saying at a basic level. This seems obvious, but it is always worth making a quick mention of what the source says 'on the line' and then comparing it with your own knowledge of the subject. Then you need to demonstrate an understanding of what the source is telling you indirectly by reading 'between the lines'. Here you should make clear and concise inferences about the source, using phrases like 'the source suggests that...' or 'it is implied that...' It is even better if you can work out what the source is not telling you; whether or not there are omissions or weak points in its argument. Finally, you need to assess the provenance of the source by using the information given to you about the author, date and origin of the source to make a judgement about reliability and utility (usefulness). Again, use your own knowledge of the topic to analyse the source's reliability and judge whether or not it is useful to the historian investigating the topic in question.