When you're studying history it can be tempting to memorise all the facts in your textbook but at A-Level facts will only get you so far. A-Level history exams examine your ability to examine the key themes relevant to the question you're doing. When revising think about the themes you might be asked about in relation to the period. Think about issues like social reform, political structures and economic developments. Remember that the mark scheme wants a specific approach and make sure that as much of your revision is focussed on the kind of essays your exam board sets. Best of all is to do lots of practice essays in timed conditions, it feels frustrating to write pages that you're just throwing away after the exam but it will make you comfortable on the day of the exam if you know you've already answered questions like the ones in front of you. When I was doing A-Level history I went through all of the past papers and when I sat my exam I'd already answered questions very similar to what was in the exam.