I would suggest, first thing to do is reading the books of our own interest. While reading, there comes so many terms and words which we don't understand, for that consult a dictionary. This will helps to improve vocabulary. Secondly start writing diaries or anything on daily basis and use that vocab correctly in a sentence. Once you write, proofread it, check your spelling mistakes, make sure to add appropriate prepositions and vocabulary. And lastly practice more and more. As it is said rightly practice makes a man perfect