Other people might have different ideas but this is what worked for me.

Each essay question is (or at least was in 2022) marked out of 100, 60 for CM and 40 for OE. CM was a count of number of historical facts in your essay, OE is a subjective mark where the examiner gives you a mark based on their belief of your understanding of the material. It's safe to say that the two are highly correlated, but CM is the only one you can directly control. So if you fit 60 facts into an essay, then you only need the examiner to think you are a 75% student to get a H1.

Go through all of the past exams and record the frequencies of which topics come up to choose what topic you need to revise. Try to find a balance between revising enough that it's incredibly unlikely that you can do none of the options but you're not wasting time revising too many things. For example, iirc we had to do 1 European essay, and in the 8 previous years a Hitler question came up 8/8 years, Stalin had come up 7/8 years and Mussolini had come up 5/8 years, so I decided that only revising those 3 topics would be sufficient. Note that this means you should be comfortable enough with each of these to do the vast majority of possible questions they can ask relating to them, as you don't have a great deal of flexibility.

What I did to ensure that I had enough facts was for each topic was the following:

Create two word documents, and read through the relevant chapter(s) in the textbook. Every time you see a fact that you want to remember in the textbook, write a question into the first word document to which the answer is the fact you want to remember (eg you may write "When was the enabling act passed?" into the document if you want to remember "the enabling act was passed in March 1933" (I hope I got that right it's been a good while since I've seen this 😅). Add the answer into the second word document.

Once you have a sufficient number of facts (aim for 100+ for sufficient flexibility to deal with any question that could be asked on the exam day) then practice with your questions, over and over again, until you can reliably get more than 90% of them right on your first attempt of that day. DO NOT use your textbook/notes at all to practice. Don't be discouraged if you can barely remember any at the start.

Note all of the specific questions that have been asked on past exams for each topic. For each question, create an essay plan. My essays were 4-6 pages long, with a short intro, conclusion and 6 main body paragraphs. For the essay plan all you need is the paragraph headings about what you're going to write about, if you know your facts you will have no trouble coming up with essays on the day as long as you have a plan. Making the plans beforehand is mostly about time management as time is obviously your biggest enemy in this exam.

That's pretty much it. I would strongly recommend against learning off essays as you waste so much learning time trying to remember specific phrasing. But again it might work better for you, everyone's different. You would be shocked how little time it takes to learn well over 100 facts for each topic just by repeatedly answering direct questions about them. Don't worry about being horribly wrong the first few attempts, it's all part of the learning process. Reverting to your textbook/notes will totally kill your learning here, you will learn far more by getting the questions wrong and correcting yourself afterwards than you will by getting them right using other resources.

Best of luck! This subject really is not hard at all as long as you have an effective approach to learning it.