How do I know how to structure my response to an essay question?

Structuring a response to an essay(or source) question in A-Level History is considered one of the most difficult parts of the subject at this level. However, if students can go approach an exam, or even a practise question, with a general idea of how essays are best structured, they can easily scaffold top-level answers to any exam question that the examiners might throw at them.
The most important things to show when writing A/A* essays are clarity, flow, impact and reflexive thinking. For a general essay question(usually in Part B of most exams), students are provided with a kind of statement to debate or analyse the 'validity of'. My suggestion would be to immediately brainstorm all your initial, default thoughts about the question- any opinions you hold with regards to the statement, any important facts, dates or statistics that strike you as relevant. You might even have a more tempered, high-level style opinion on the statement before you've planned your essay. For example, something like, whilst it is true that..., it is perhaps less valid to suggest that.... . These initial, immediate thoughts are crucially important, and are usually 'right'. Once you have brainstormed and collected your initial ideas on the question, however fragmentary and in-cohesive they may seem at this point, you can move on to formalising a more solid, impactful plan for writing.
Above all, an essay plan should set out your line of argument clearly and follow a structure demonstrating logical progression. Unlike in English Literature A-Level, for example, where students' opinions on a statement often change and have an epiphany half-way through writing the essay(and this is acceptable), in History, students should think of essay writing as an honouring of their initial line of argument, avoiding LofA 'mutations' mid-way through. You shouldn't be deciding to pursue a different argument mid-way through writing; the directions you intend to take should be clear and inherent in the opening lines of your introduction. However, as a student, I found that the best way to get around this undeniable constraint was to allow myself a 'challenge' or 'reflexivity' point towards the end of my essay. For example, if, when reading the statement, I could separate the question in to around three sections(or paragraphs), but something still didn't quite add up, I would often take this route. For example, it may be that something, even on the level of the wording of the question, has been set up as deliberately provocative by the examiners to probe students at the top-levels. In the final paragraph before your conclusion, you may want to challenge the wording of the question itself, or use this to 'round-up' any misconceptions inherent in the statement that you have not had the time nor place to deal with anywhere else in the essay. These subtle judgements often don't correspond to any 'section', and that is why examiners only expect top-level students to attempt to find a way of challenging them in their essays. Of course, this should never be forced, but when done effectively, it can elevate a response to the highest of levels, showing astute powers of comprehension and precise historical judgement.