How do you structure an answer to a sources question?

First and foremost, you must look at the weighting of the marks for the question. You must check markschemes and examiner reports for the specific board to see if the marks for the question are more heavily weighted on your own knowledge, or the source so you know what the examiner is looking for and what specifically to focus on. 

However - as a general note for sources questions, you must lead with the source. This means that you should start paragraphs with a direct reference to a source. For example:

'Source A states that "......", thus disagreeing with the statement in the question. However, we can see that there is a clear element of bias against Cromwell as it is written by a royalist, thus reducing the reliability of the source'

However, most sources questions do depend on some own, background knowledge of the era. This must be used to back up and coroborrate the source. For example, in this instance, we can use our own knowledge to deploy examples of specific case studies of royalists going against Cromwell or the Protectorate as evidence that Royalists were opposed to Cromwell. Here, we have shown that we are using the source to make a point, and backing it up with our own contextual knowledge. As a general note, two facts of own knowledge should be used to back up each point made in the source. 

It is also important to coroborrate the sources. This helps with stucturing the body of the essay. I would recommend jotting down/highlighting in different those [parts of the] sources that agree with the question, disagree with the question and those which are somewhat ambiguous. Once you have done this, it will be easier to group sources with similar stances together in the same paragraph, rather than focusing on each source individually and exclusively.