How do I effectively revise for an essay question?

Remember here that there are two things that examiners like to see: knowledge and skill. Focus your revision separately on these two things and then bring them together to give a great answer.
To revise knowledge, it's really useful to read over your texbook and classwork to give you a good overview of the topic - don't worry too much about the detail just yet. Divide your topic into sub-sections and see how much you can remember about each one. I find mind maps really useful here, but you might prefer another method such as bullet points, or timelines. Now you can check anything you have missed/weren't sure about in your books and add it to your revision notes in a new colour - this will help you to remember those trickier facts you forgot the first time. Underline key dates/names/acts in your new notes to create a solid, condensed list of important own knowledge you can test yourself on.
Revising skill is all about the way you answer the question. The best way to revise this is to look at as many past exam papers as you can get your hands on! Familiarise yourself with the types of questions they ask. Very often, you'll be asked to say if you agree or disagree with something. Ultimately, you need to persuade the examiner that your opinion is valid, using the knowledge that you've revised. Challenge yourself by practising writing answers for both sides. This way, you're revising the skill of persuasion rather than simply listing knowledge that supports your answer.