1. Read:
This is the best in quality as one sees the usage and the context. However, it’s not a direct method for vocab improvement and the ROI in terms of new words learnt for an hr put in is low unless one’s vocab is less than average. This is assuming the average Indian English newspaper. Plus, there is additional bother of looking up a dictionary every time.

2. Vocab books like Norman Lewis and Rosenbaum:
These books look at the roots of the words, origins etc. Thus, each word has a history etc. very easy to pick up certain words and also helps to guess the meaning of a word using its roots even the one had never seen it until that moment. Downside, there are only a few words with a story. Also the roots can give misleading results at times.

3. Word lists:
Every coaching/prep book has them. Very direct approach and has a high ROI. But it looks a monumental task to finish a few thousand words. Can be done with regular effort and tools such as flash cards. The key is to learn the usage and not to memorize the words.

One can choose depending upon one’s needs. If you find it difficult to get started, option 2 is probably the best. If you’re pretty strong already reading and combing through word list may be a good way. If your English is particularly weak, read and do the word lists. Option 2 can be a support.