I can never get the hang of an essay. How can I write a good one?

There's a formula for that: PEEAOB is a structure for every paragraph you write (and an essay is a few of those paragraphs with an introduction and a conclusion acting as bookends). The interesting thing is that with a few tweaks, this formula can be adapted to any humanity subject and to any wordcount, but it works best for History because you mingle content with interpretation at every stage.

The introduction mirrors the conclusion: it has to contextualise the question, signpost your final answer (also referred to as your thesis), outline the perspective(s) you will be looking at, then acknowledge any weaknesses/challenges to that stance and what you plan to do to overcome those. The introduction must not be longer than 300 words. 

Now for paragraphs. Every sentence - and indeed every word you use - has value and is vital to the point you're making. Try and stick each step below into one sentence.

P: Point. State the point of your paragraph. Others may call it a key sentence or a key argument. It must be kept short and clear.

E: Evidence. Why is your point valid, and on what grounds? Be clear and precise in how you state your proof, give dates/names/facts if possible.

E: Explain. Why are you making this point? How does it link back to the question? (Tip: the phrases "This means that..." or "This happened because..." are perfect for explaining something). Use a quote from a historian to support your point.

A: Analyse. Why is this point relevant? Is it completely accurate? Can you link it to others? Who else says what you're saying? Are there any people who directly disagree with this point? Who, and why?

O: Own opinion. Golden rule: never say "I think" or "Personally, I...". Always use the impersonal form, like "It seems that..."; or "It can be said..." Give your opinion on that particular point, but keep it aloof, pithy and original: don't just repeat what you've already said.

B: Back to the question. You've just had to illustrate and explain a point, which means you've inevitably strayed a little bit from the question. How can you link it back quickly and succinctly?

The conclusion is the most important part of the essay. It brings all the points back together to form the thesis you originally decided upon, so MAKE SURE IT CORRELATES WITH THE INTRO. Take it a bit further by looking at wider horizons: have the effects mentioned in your thesis had other consequences? Is your thesis controversial? Can you link it to a similar modern issue?

Above all, AVOID NARRATIVE. History isn't a series of stories, it's a commentary.

And don't expect magic results. Essay-writing and paragraph structure are skills that need much practice.