Hello,
A good range of vocabulary is essential for a high band score in the speaking and writing sections of the IELTS exam as 'Lexical Resource' is one of the assessment criteria for both sections.
So what is the most effective way to learn new words? Certainly not memorising tons of words from a dictionary!
1. Develop a daily reading habit of at least 30 minutes. Read the newspaper, a book or magazine, even online sites if you wish, but make sure you read on a variety of topics.
2. While reading, mark the new words, and learn the meaning from a good learner's dictionary. Oxford Learner's dictionary is a good site to refer to. This way you learn the word meanings, usage from sentences as examples, its forms, synonyms and even use in idioms. List out 10 new words daily and write them down along with the meanings.
3. Identify word combinations or 'collocations' while reading and note them as well. To achieve a band score of 7 or above, collocations are important, especially in writing tasks.
4. Learn synonyms of the new words from any thesaurus.
5. Every 3-4 days, revise the word list.
This way, you learn new words in a context and are more likely to remember them!
Good luck with the IELTS exam.