The most important thing about structuring any essay is knowing what you think about the question! Before you work on the specifics of what goes in which paragraph, try to think of a short, one- or two-sentence answer to the question you've been asked. This is your overall argument. Keeping this in mind as you write will keep you from waffling and make your structure much more focussed.
In terms of the structure of the paragraphs, there are several ways you could do this. Some people like to split their essays into themes or topics (so if you were, say, evaluating why Germany lost World War II, you might want to split it into sections like international relations, economics, military technology, domestic morale).
Another good way is to organise your essay by argument. You will have done this before in simple “for-and-against” essays, where you argue two sides of the question and then conclude. For A-level you need to make this a little more sophisticated- if you can, bring in other historians’ opinions. For example, a really good essay structure is to explain why a particular historian or group of historians think a certain way, explain how these views could be challenged, and then put forward your own opinion on the issue. This structure has the advantage of keeping you focussed on the question rather than adding in unnecessary description, and evidence of independent thought and knowledge of historians’ views should pull you up into the higher grade boundaries
Both structures are equally valid: you will find that certain questions and your own writing style might lend themselves to one or the other approach. Whichever you choose, the important thing is to keep things clear and simple: focus your paragraphs with a Point Evidence Explain approach (or something similar) and remember that every piece of information you put in has to be relevant to answering the question.
The more students I tutor at 'A' Level the more I can see that they are usually conscientious enough to learn about topics and complete all the tasks set. Their essays are well set out in paragraphs with good English and yet they seem to get trapped at Grade C level or below. There is something missing - normally the ability to answer the question that has been set. Often their essays turn into a knowledge-based description of a topic, rather than answering a question asking, 'How far...........' or 'To what extent do you agree...........'. If you do not answer the question properly you are destined to miss out on a higher grade. The questions are set this way to see if you can access the higher skills of analysis, evaluation and making a reasoned judgement or conclusion.
How far.....' questions imply the need for a judgement by the student. Judgements are not made in the final paragraph alone. The judgement should be known to the student at the start of the essay and the rest of the essay should provide the evidence for reaching that judgement. The student will analyse and evaluate the evidence used throughout the essay so that the final paragraph comes as no surprise to the exam marker. At the top grade you will see examiner comments about the argument being sustained throughout the essay.
Most of my tutoring at this level is to help students discover ways of accessing these higher skills. Often it means finding a better essay writing method with analysis and evaluation at its core. Some of my students have been very successful at adapting and adjusting their essay stles to maximise their grades which makes my work very rewarding.