In order to answer this in an essay format, the student has to first critically analyse the question. What is meant by success and by failure? A success or a failure in who's eyes: Lenin's, Trotsky's, Stalin's, the West? These types of critical ways of addressing a question can show that the student is actively engaging in answering the question on a more intellectual basis.
In the introduction it is important to define the thesis of your argument. However, before you define this argument, it is essential to give the key dates which you will be looking at, so in this case from 1917/18 to 1923 (Lenin's death). Coming to the thesis, you need to outline your argument. If you did think Lenin's foreign policy was a success, then describe which policies in particular you relate a successful outcome to. If you thought it was a failure, mention those policies that would define Lenin's foreign policy as such. It would be good to have at least one counter argument in your thesis, stating that not all of his policy was either a full success or failure.
The first two body paragraphs could discuss the success of Lenin's foreign policy. One could look at the 1918 Treaty of Brest-Litovsk as successful foreign policy. The general public wanted the devastating war to end, as it was bringing famine, death on an unparalleled scale, and foreign invasion to Russia. Therefore, for the Bolsheviks, this was seen as a success, as it allowed the country to be relieved from suffering, and especially after the Revolution, allowed some political order to be restored. It is in these moments that you can demonstrate the ability to look at the question critically. In order to counterargue your own point, you could mention that the Mensheviks, or the Western powers trying to defeat Germany and Austria-Hungary, saw it as a failure, as it allowed the balance of power to shift, giving Germany an advange, now that it no longer had to fight on two fronts.
This type of argument could be continued with other successful foreign policies such as the Treaty of Rapallo of 1922, which let the Red Army get trained by German soldiers, and allowed for full diplomatic relations to resume.
In terms of failure, you could decide which foreign policy can be seen as such. A valuable example would be the set up of the Comintern (Communist International), which was initiated in order to ignite communist revolutions globally. The argument could provide evidence by showing that it was not until 1949 in China that another successful communist revolution occurred, which was long after Lenin's death. Thus, the foreign policy goal was not achieved.
There are several examples you could give, including the Civil War, but in the string of the argument it is important to distinguish, why these policies were a success/failure, and also for whom they succeeded/failed. Most importantly, you need to remember to use historiography in order to support these claims. It is not enough to quote historians, you also need to engage with the historian's theory and either agree or critique it in terms of the question. Good historians to research for this type of question would be Orlando Figes and AJP Taylor.
For the conclusion of the essay, you should stay concise, and not bring any new ideas in. You can rephrase your thesis statement made in the introduction. Then, bring in your main argument (in a sentence!) for each body paragraph, and summarise what your argument was, and how the different policies were a success/failure. Lastly, to conclude the essay, write a strong sentence with a final thought or a final answer to the question in one go: i.e. In conclusion, Lenin's foreign policy can be seen as a success in terms of Bolshevik aims, as it achieved to end Russian involvement in a capitalist war, resume diplomatic relations after the revolution, and allow Russia to focus on its domestic reconstruction.