In answering this question you must make a judgement as to how important WW1 was as a factor in bringing about the October Revolution. To do this you must also look at other factors, such as the long term problems of famine and peasant discontent in Russia, the February 1917 Revolution, which overthrew the tsar and the actions of the Bolsheviks. Even if you feel that WW1 is the most important you must mention other factors and explain why they are less important. If you are unsure about which factor was most important then it's helpful to look at the key events of the October Revolution and what led up to it. If you think that the October Revolution couldn't have happened without the tsar already having been overthrown then say so and explain why. Having a clear idea of what you think is the most important factor is helpful as it gives you something to drive your essay forward. While they do take up a little bit of time having a basic plan is really helpful in case you get stuck or run out of ideas.
In the introduction you should set out your argument, in this case to what extent WW1 was responsible for bringing about the October Revolution. It's important not just to state what your view is but also to back it up with evidence. For example, if you feel WW1 was primarily responsible you could say it was because it worsened existing problems such as famine and undermined autocracy through the tsar's failures on the Eastern Front. If there are any terms in the question which seem unclear it is useful to explain them in the introduction as well. To show a strong understanding of the topic you could also briefly analyse the question itself. For this question you could examine whether the October Revolution was even a revolution or just a Bolshevik coup and state how this effects your answer. You don't have to spend a lot of time on this, just a couple of sentences would be fine. It's just a good way to show you've really learnt the material.
Try and structure your essay so each paragraph covers a different topic. Avoid looking at the question chronologically as this can make the structure of the essay very unbalanced. Having a brief outline of what each paragraph will be on in your plan, even just a word, is very useful. Don't worry though if you go off your plan a bit so long as you're still answering the question clearly. The plan is there to help you but it's just a guideline. If you realise as you're writing your essay that you had forgotten an important event that you think needs mentioning then just put it in where you think it fits. The most important thing is that your argument is coherent all the way through. Referring to the question throughout is useful both for you and the examiner as it reminds you both of what your argument is. Support your argument throughout with any facts or figures you think would be helpful. For example if you think WW1 was key because it worsened the famine then explain why this was the case. If you know any statistics, such as grain production figures, then use these to back up your argument.
Your conclusion is key in summing up your argument. If you are running out of time try to just even write one sentence for a conclusion to draw it all together. Try not to introduce any new ideas into your conclusion, just outline your key points and show how these support your argument. If you think that you went a bit off topic or got confused in the main body of your essay then a clear conclusion can really help to explain to the examiner just what your argument is.