To what extent was the Soviet Union's economy the most important factor in their victory in WW1?

When answering a question like this I'd start by breaking the answer down into sections; some possible sections:

• The economy
• Stalin's leadership
• Hitler's mistakes
• Other factors, such as the weather, the Western Front opening, the Russian people etc.

Now, it's important, given that it is mentioned in the question, to address the economy. I'd do this immediately after the introduction (you're introduction should outline your key argument, as well as introduce a couple of contrasting viewpoints, this will be more effective if you can quote a historian). Each paragraph of your essay should address each side of the argument, with as much detail as you can muster, for instance some estimate that as much as 1/3 of Russian military equipment was provided by the lend/lease system. After outlining the key arguments you should pick a side, for example you may argue that because Stalin was an absolute dictator the strength of the economy is simply an element of his leadership rather than a factor in itself, or that Hitler's mistakes were much more important as the Russian economy was weak (evidenced by its near collapse in 1942).

I would then move on to the next most important section, so if you've argued Hitler's mistakes were more important than the economy then that should follow the economy. You should follow a similar structure as the first paragraph: loads of specific detail and a smattering of historians' quotes and then you should follow the same argument structure as before. It is important that you don't suddenly change your opinion and begin saying the economy is more important, but you can say that some historians might argue that.

The final paragraph should deal with the most important factor you have left in the same structure. It is worth mentioning the other factors, but as they're less important generally you shouldn't spend too much time on them.

Then conclude. A conclusion should mirror, but not repeat, the introduction. Sum up your argument and re-cap a little of why you have decided on a that particular argument.

Final thoughts:

• Quality of written expression is always important.
• Dealing with historiography is especially important to get the highest grades. Try not to stick to the course textbook for this, read widely and quote widely.
• When adding detail try to avoid simply listing facts. You will get better grades if you can integrate it throughout your argument in a natural sounding way.