Remember being taught how to outline a paper when you where in school? You had to write a thesis statement that identified the key purpose of your paper. You then outlined an introduction and developed carefully constructed supporting statements listed with roman numerals that led to the all important conclusion.
Oh, how my creative process has changed. There are occasions where I may begin at the beginning. Sometimes I begin right in the middle with something I want to explore. I won't have a sense of it's significance yet, or the direction I will take in writing about it. It's just an idea that needs a thesis and a conclusion, among other things, to turn it into a useful piece of writing. Other times, I begin at the end. My conclusion isn't just a recapitulation of the thesis; it's "point B" in the task of "getting from point A to point B." Beginning at the end requires that I work backwards and discover the material my creative process has unconsciously been working on.
If I'm having trouble writing something, often it's because I'm trying to start in the wrong place.
Sometimes a section of a piece is elusive. An introduction that doesn't start off nicely, a transition that won't flow, or a conclusion that isn't conclusive. The middle or the conclusion is often a better place to start in these situations. Even if I don't know the end when I begin writing, paying attention to the order in which I write things makes for happier endings. At least for the writer.