Now the first question that might come to your brain is from where should you learn new words. Those who are going through tests some other tests (besides IELTS) for which a large number of words are generally needed to be memorized will automatically have a huge collection of words. But knowing such a giant list doesn’t mean one knows to articulate the mugged words in correct manner, without undermining the literary flavor of the writing. Generally based on this author’s observation the words people memorise for these type of exams are commonly used in the articles of some of the popular international news medias. While you read those articles simply make a habit of spotting new words and try to see how the writer have used the word. You may wish to copy those sentences for your future reference.
On the other hand if you don’t require to mug up words for any exam or you simply don’t like the task of memorizing, simply make a habit of reading quality articles and see the usage of variety of unknown or less known words.
If you have a very limited time to prepare for IELTS, this author would suggest not to try learning new words much, unless you have an extra sharp memory to learn new stuff in short time and apply it in correctly in sentences. In such scenarios simply use the vocabulary stock you already have with yourself and its probably unwise to underestimate it. Just be careful to avoid excessive repetition of same words or phrases. If the flow of writing is coherent to the task and have the display of intellectual capabilities in the writing, it should not be difficult to get your preferred scored.
However, language preparation is grossly subjective and experiences and performances vary from person to person. If after trying your best you are not being able to get your desired score, you should probably seek help of an expert.