The most important thing to remember when you write an IB History essay is to answer the question. This sounds simple, but the majority of people lose most of their marks by drifting off topic (if you don't address the question, it's impossible to score more than 40% on a history essay).
The next thing to consider is the title. What is the main command term? What are the implications of the question? What is the date range? The have to ask these questions in order to really pick apart the title and understand what it wants you to do.
The last thing to consider is structure. Paper 2 only allows you 45 minutes per essay, so you won't be able to write more than three or four main paragraphs (not including your introduction or conclusions). This is where planning is really important - it is your opportunity to structure your ideas cohesively and form links between your paragraphs.