Most of think (erroneously) that writers just sit down and churn out a wonderful essay, story or poem in one sitting in a flash of genius and inspiration. This is not true. Experienced writers use the writing process from start to finish to help them write a clear document. If you do not reflect on your composition in stages and make changes as you develop it, you will not see all the problems or error in it. Don't try to write an essay or story just once and leave the room. That's a mistake made by novice writers and will be glaringly obvious to an experienced reader. Stay and look though your work. Reflect upon what you've composed. Even better, use a writing process when you pre-write and rough draft, organize ideas, edit and proofread. Your writing will suffer the consequences of poor craftsmanship otherwise.