How should I structure a comparative essay?

There are two main ways in which this can be done.The first is to write about the texts one after the other, analyse one in full in relation to the question focus (e.g. war), and then move onto the other. Many find this method easier to fully flesh out a text and get it all out at once. It's true that this can make it easier to access a deeper level of analysis as you have full concentration on one text, and many examiners don't mind students doing this. However, this method puts more pressure on the second text analysis to make all the links and comparisons between texts by referring to earlier in the essay, which can make it seem as though the essay was not properly thought through with ideas added along the way, unless done impeccably well.
A generally preferred method by examiners that creates, in my opinion, a more fluid essay, is idea based. This involves talking about an idea you've come up with in relation to the question (e.g. 'war is fruitless',) linking it with both texts and then having a mini conclusion - all within each paragraph. Many students struggle with this as involves more planning at the beginning of the exam time. I personally use diagrams to plan. (I'd show the student a diagram at this point to help them plan their essay) However, if you practice it can result in higher marks as your essay automatically appears well thought through and coherent with constant comparisons between texts, which is the point of the question!