I have found that ideas come to you in the most unlikely places and like the 'Eureka' moment, I have many whilst in the bath. Some of them obviously can't be published, but others at least start out there.
Sometimes the spark can be as little as a phase or a comment and that starts the creative juices flowing
Writing it down
I find that I can't plan a story. I've tried, God knows I've tried, but I just seem to have the idea and start writing.
I write on computer, so it's easy to chop and change things around, but usually, I start writing a scene from the initial spark or ideas.
Sometimes characters grow out of that and other times, it's like descriptive stuff only.
I have been told "show, don't tell," but basically that's bollocks. The story comes and however it does, if that's what you're comfortable with, then that's the way it should be. I think the closer it comes to being from the heart, the better it will be.
Sometimes, stories just happen. You start and it naturally flows from one thing to the next, through the twists and turns that just pop into the head.
Other times, its a real pain and you have to mull over things for ages or just say goodbye to the idea and go on to something else.
I have many stories that have ended up that way.
These are what normally end up writing my stories for me.
I don't know how they do it, but I almost feel as though I'm just there for the ride; as a dictation machine perhaps and all I do is let them lead me around by the nose until the story gets finished.
Most often though, I have to take a firm stance with them and curtail their meanderings otherwise, the story would likely as not, wind up somewhere other than where I wanted it to go, or fast become War and Peace's big brother.
Characters have to follow certain rules.
They have to stay true to themselves otherwise the stories can take on an unrealistic twist that can and often does lose the reader's attention.
So pay attention to how your characters react to stuff, the language they use and the way they are with others.
These are the bits you try and follow - or get your characters to follow when you're busy beavering away at the keyboard.
This is the story itself.
This is like the container that the subplots all swim around in.
Your characters will sometimes want to do things for reasons other than what's obvious and they will have their own agenda.
It's wise to keep a handle on these.
Admittedly, my stories are usually quite straightforward at present, but I am growing in confidence all the time, growing in ability too and more able now to put more meat in my story crafting.
Remember, you are asking to reader to believe in what you're writing most of the time, so things have to be believable. They tend to lose interest otherwise, but as you become more adept at writing prose, you will find out what happens when you get too wordy, not descriptive enough or your plot leaves too many lose ends.
Proofing, beta reading and editing
This is a must.
If you do nothing else, get your work edited or at least beta-read.
This way, you will find out from a readers perspective, how your story flows, how the characters interact and whether the plot is believable.
I used to be very possessive of my work, but then I found that others have their uses. They can see the bits that haven't come out of my head and landed on the paper (or virtual paper anyway); those bits I know, but may not have said or even implied.
You don't have to listen to them, but it is wise to get a second opinion.
In addition, word processors are cleverer than they have ever been, but don't always spot a word that shouldn't be there or has been misspelt, though is still a word. Another pair of human eyes are worth their weight in gold for this.
Sometimes things happen for no apparent reason, but you should know, readers are a canny lot and will often spot things that you don't see as important.
Loopholes in the plot can often leave the reader feeling unfulfilled and dissatisfied with the work you have so painstakingly put together, so be aware - use an editor.
However you finish your story, be comfortable with it.
If you can, leave it for a week or so - longer if you can wait, so that you don't automatically know what's coming next. You will find that mistakes you made will stand out more readily; typos become clearer and things that don't fit or don't flow will also be more obvious.
Don't be too quick to submit.