Every author has their own method. I haven't written too much long-fic myself, but the times when I have, my technique has been to plot out the basic outline in my head, start writing, get confused when the story suddenly veers away from where I'd thought it was going to go, and then write and delete several pages (often at the same time) until I end up at an ending that may or may not resemble the one I had planned.
Your experience may differ. *g*
And yes, getting a BR to go over what you have is very helpful in so many ways. You can choose whether to work with one while you're still writing, or write a first draft and send it to them when you're done. Your choice.
