I like to write with an outline handy. Because in order to really get someplace in your writing, you need to have an idea of where you want to go.
Case in point: I am sitting down to write this blog without a clue of what I want to say. Oh, I have a few ideas. We went on a temple trip today with the youth in our ward; I could write about that. We are now doing more cleaning in our basement. There's not much interesting about that, but I'm sure I could come up with something. Our dog caught a mouse in our basement. Now that was interesting. Especially the fact that there could be more of them. I shudder to think of a nest of mice helping themselves to our food storage. If we'd packaged it more carefully, we wouldn't have to worry about it because they couldn't get in. Instead, we have a few bags of wheat that we always *meant* to move to plastic airtight containers, but we're always finding things that are more important (or more interesting) to do than repackage wheat.
Well, anyway. I did not intend to blog about mice or food storage today. But my free writing kind of led me there. So that's one thing that just sitting down and writing will do for you. Don't wait until you have a Pulitzer-worthy idea, and don't wait till you have every detail mapped out. Just start writing and you'll be amazed at what comes.
Pay close attention and you'll notice that my thesis statement for this blog does not match my conclusion in the previous paragraph. Did I change my mind as I wrote? Sometimes that happens, especially when you write without a plan in place. And sometimes it just happens anyway.
So then you'd better go back and edit what you write after you finish. Because in the end, you really do want your writing to look like you knew what you were doing the whole time.