This is a very common problem for people who are learning English as a second language. The question is how do you do it? During my experience in the English teaching industry I believe that I have found a method that can work for many people.
I like to call it the 'Paraphrase method'. It is actually very simple to execute and I know that many English language students have already achieved through this method.
There are five stages to the process.
1. Article - You first need to choose a suitable article, letter, report etc. Actually any body of writing will do. Be sure to choose an article that suits your level of English.
2. Decide - When you have chosen your article you then need to choose a paragraph of writing from the article. As above choose a paragraph that suits your level, don't make the mistake of choosing something too hard. Don't choose something too easy either.
3. Paraphrase - Next you have to paraphrase the paragraph. To do this I would suggest you have an English - English dictionary and a Thesaurus available to help you. The thesaurus will help you find synonyms of the words you want to paraphrase.
4. Sentences - Now make your own sentences with the new words. This will check your understanding of these new words. If you have an English teacher then check your sentences in your next lesson.
5. Practice - In the final stage you practice using your new words. This means actively using them in everyday conversation as often as possible. I would recommend using the words a minimum of three times each day for one month.