Hi Anu! Now that you have around 5 months to go for CAT and other MBA entrance exams, you can actually plan your preparation properly. The best way to prepare for vocabulary is to read as much as possible. Read anything you can lay your hands on, be it novels, short stories, newspapers, magazines, anything. Select the words you don't know the meaning of and write them down. Take up a dictionary and write the meanings and usages next to those words and revise them. Also, practice them by using them in writing and speaking to bring more familiarity. 

Books - Refer to the books Word Power Made Easy by Norman Lewis, 1100 Words You Need to Know by Barron, Merriam Webster's Vocabulary Builder, Thesaurus etc. Follow the same strategy to learn the words and use them in your sentences. 

Use flash cards - Write down the words and meanings in different cards and put them in your room or anywhere where you can see it frequently. In this way, you would be able to memorise a lot of words without putting much effort. 

Practice - Take sectional practice tests where you can select vocabulary related questions and solve them. After solving each test, check the answers or get them analysed in order to improve your learning skills. 

Hope this helps! All the best!