If you ask a random person how to write a book they'd probably say something like "start at the beginning". That advice, if taken, would lead to three possible outcomes:
1). A best-selling novel (unlikely)
2). A jumbled mess of incoherent thoughts (much more probable)
3). You staring at a blank screen wondering what the hell you were thinking in the first place (almost a certainty)
Let's face it, you're not going to sit down and just start typing "It was the best of times, it was the worst of times". Well, maybe you will but Mr. Dickens already did that one (and that line is amazingly insightful) so think again.
If you ask someone who's actually written a book or who is in some sort of "writer's group" they'd probably tell you to make an outline, then profile your characters, then put tab 'A' in slot 'B' and viola! You've just constructed a bookcase. Er, a book. Okay, be fair; that is more like assembling furniture than writing a story.
Stories, whether long or short, begin with an idea. You can't simply decide to write one and expect it to happen. If you don't have the idea you don't have the kernel on which the story is built. That kernel may be a situation or it may be a character, but you have to have some inspirational point to start from.
"What would life be like in this situation?" is an example of the former starting point. "What if there was a person like this?" an example of the latter. You will of course need both: either you have the scenario and populate it, or you have the population and drop them into the scenario. Both will evolve as the story unfolds.
Then you start writing. Not at the beginning, although it may seem like so, but at some part of the story which really stands out in your mind: the scene where the lovers first kiss, the part where the hero saves the day, the tragic moment when the beloved character dies. Something that will act as a catalyst for what comes before that time and after it.
You will write not from front to back nor back to front. More like from the middle to the two ends. And then you will go back over it. And then you will go back over it. And then you will go back over it. You may end up going back over it too many times, the way artists sometimes over-work a painting.
You're looking for plot holes, details, little things that need to be mentioned early on so that things that come later make sense. When first written character 'A' meets character 'B' and it's a situation that leads to further interaction between the two. Later on we find out that character 'B' has attributes that should have been noticed by character 'A' at their first encounter. So it gets rewritten:
First Draft:
"Amy looked up at a blonde woman with pale skin and a rabbit-like expression of timid fear on her face."
Later we discover the woman is an albino with very sensitive skin that dictates certain aspects which should have been apparent from the start. So we add information.
Rewrite:
"Amy looked up at a small, very blonde woman with extremely pale skin and a rabbit-like expression of timid fear on her face who was for some reason wearing satin pajamas that were clearly too big for her."
And it may get rewritten again (and again) before it is satisfactory to the story as a whole. You're probably not going to get it exactly the way it needs to be on the first try. Or possibly even the eleventh. Putting in every necessary detail at the right moment is important. So is leaving out every unnecessary detail wherever they may land. But you've got to start somewhere; just don't fixate on it being the beginning.
You've also got to end somewhere. Writing can too often be like painting the Golden Gate Bridge; just when you think you're done, it's time to start over. Classically stories have a beginning, a middle, and an end. The middle is typically defined as the climax of the action, and isn't in any other way the actual midpoint. When you've 'completed' your tale there is a terrible impetus to ask yourself "then what happened?" and get trapped in the world of sequels and prequels - authors' hell, but publicists' heaven (milk the franchise for all it's worth, then keep on doing it).
If you're thinking this approach will lead to a disorganised mess, you're not wrong. The cut & paste process of putting the paragraphs in the right order is tedious and time consuming - and may take longer than the writing. Fortunately computers allow us to have multiple files open at once, so rather than mess up the source material you can edit it into a new space and hopefully get it in the right order. An outline may help you here, as well as parenthetical (bracketed) notes to yourself about what is going on when in the original text. Anyway it's better than the old fashioned way of literally (pun intended) cutting the written work with scissors and using library paste to stick it on to a clean sheet in what is hopefully the right order. (Didn't you ever wonder where the term "cut & paste" came from? Now you know.)