Practice makes perfect is an old adage which is perfectly true. PRACTICE, PRACTICE, PRACTICE!!! will lead to PERFECT.
Just write an essay (different topic) a day, continuously for about 2 weeks. Simultaneously, enrich your vocabulary, idioms try to incorporate in the essay.After 2 weeks, write the essay for the topic which you did on first day. You could see vivid changes in organization, way of writing and more.