Intro: define 'most important' - for example which factor had biggest impact, caused most economic upset or the most political turmoil. Show awareness of other potential causes. State your line of argument (whether you agree/disagree with the question).
Your first paragraph should discus the factor in the question: in this case discuss the Treaty of Versailles. Use the 'PEE' structure: Point, Example, Explain. Always link back to the line of argument you stated in intro - if you said Versailles was the most important in your introduction use this first paragraph to explain why. If not have a linking sentence at the end of this first paragraph suggesting, for example, that Versailles only set the scene for Germany's problems and that other factors were more important. Go on to consider the other factors/causes (e.g. Germany's invasion of Poland, appeasement, the Great Depression) following the same PEE structure as your first paragraph. Always link each factor back to you original opinion. Conclude by summarising the points you have discussed and reinforce your opinion.