How should you go about answering a reading comprehension question in a GCSE Latin Language paper?

First of all, you should bear in mind that a reading comprehension question does not require you to actually translate a sentence or passage, but rather to show knowledge of what is happening in it. Therefore, your answer does not need to be a word-for-word translation of the Latin, but more of a summary focused on answering the specific points of the question posed by the examiner. However, it can be helpful to mentally translate the sentence first and then pick out the points needed from that to answer the comprehension question.
For example, with a question such as 'itaque Romani, multis militibus suis mortuis et plurimis villis incensis, ad summum Capitolium ascendere constituerunt (lines 1–2). what two things had led them to make this decision?' you might mentally translate it first as something like 'and so, the Romans, after many of their own soldiers died and very many villages were set on fire, they decided to climb to the top of the Capitol Hill'. From this initial translation, you can now tell that that the two things that led them to make their decision were that (a) many of their soldiers had been killed and (b) very many of their villages had been set alight. Notice how picking out these two specific points is what is required by the question, rather than a full-sentence translation. However, you can offer a translation in the specific points you are making, and you don't need to paraphrase your answer too much, as you do not loose marks for close translation.