This posting from Marvin H. Swift in the Harvard Business Review, walks you through revising a short piece of text. He talks of the thinking process behind the revisions and I think it is useful to see how he questions each aspect and what it really means. In the end the text completely changes, partly as a process of making the words more clearly say what he wants to say, but partly because the structured thinking makes him see the problem differently and he changes his mind about what to say.
You too can use your writing as a way to structure your thinking. The idea is to get something down and then actually think about what you have written and what it really says. Look to see if you have explained your meaning. Look to see if you have made your argument. It is often a good idea to put it aside and come back to it later as often you will see things you did not see the first time.
Marvin H. Swift (1973) Clear Writing Means Clear Thinking Means… Harvard Business review