I believe reading is the best way to improve vocabulary, but, reading is not enough! You need to look up new words in a good dictionary like the Cambridge Advanced Learner's Dictionary, which has good sample sentences that help you understand the word better. Then, you need to keep a record in a notebook, on flashcards, or with digital flashcards you can make on so that you can review new words before you forget them! Finally, you should use the new words in your own sentences and say the sentences to a native English speaker, like me, to get feedback as to whether you are using the word correctly. Sometimes, it's easy to make mistakes when using new vocabulary in your own speech, especially when using phrasal verbs and idioms for the first time, so it's best to try it out with me before you use it on an IELTS or TOEFL test!