Every story begins with an idea. What's amazing about science fiction stories is, they often start with a cool idea. Like a spin on space travel or robots that nobody's ever thought of before. But how do you turn an idea into a story, with memorable characters and powerful moments? That's often the hard part.
Just the same way there are no rules for good storytelling (other than "tell a good story"), there's no right or wrong way to get an idea for a story. You might start out with a character or a cool scene, and build a whole story and concept around that. You could start with a world, and dream up the whole history of that world, before you figure out when/how your story starts and what the notion is. Or whatever.
But oftentimes, a great science fiction story does start with a high-concept idea — like, say, the first ever generation ship is halfway to another planet when we discover that the drugs we gave the colonists to protect against cosmic radiation are causing mutations, and maybe whatever arrives at the new planet won't be exactly human any more. (Apologies if that doesn't sound like a cool idea, I just came up with that on the fly.)
No matter how fascinating and inventive your basic story idea might be, it's still not a story. There's a term among science fiction writers and editors for a story that just lays out a cool idea and then ends: a H.A.I.T.E. story, which stands for "Here's An Idea. The End." At the very least, you want to have some progression in there, a beginning, middle and end, and maybe a few surprises along the way. And there need to be some people in there, to cope with the situations you've set up.
But you don't just want to do the very least — you want to create an awesome story, that will stick in people's minds after they're done reading it. And for that, you need to engage people's hearts as well as their minds. You need more than just a fleshed-out plot to go with your concept, you need some emotional hooks. Joy and pain, man. Like sunshine and rain.
You've got a plot. What's next?
So let's stipulate that you've taken your basic concept and figured out how it plays out. You've sketched out some of the implications, and what sort of decisions people on this colony ship might have to make about the mutations. Would some people embrace becoming a new race? Would there be therapies, to try and reverse the effects in utero? Is there any evidence that the mutations will make people either more or less suited to survival on the new planet? Does this maybe lead to a civil war on board the ship between pro- and anti-mutant factions?
Just spitballing here — one way or the other, there are twists and developments, and eventually you get to a crux, where everything hangs in the balance, and there's some resolution.
So you've got all that worked out, more or less, but it's still just a series of events. As dry as the reconstituted food waste your colonists are subsisting on (in addition to whatever they can grow in their hydroponicum.) How do you set it on fire and make it intense? How do you make that collection of events kick your readers in the hypothalamus?
Again, this is something there's no right way to do. But most of the time, it has partly to do with identifying the right protagonist (and antagonist, maybe) for your tale — someone who has needs, desires, an agenda, that are separate from the overall needs of your story. And also, it has to do with doing a spot of method acting. Or that other kind of acting, where you just project stuff. Anyway, acting. Let's take those one by one.
Who's your main character?
Well, duh. Your main character is the captain of the generation ship. Or the ship's doctor, who's trying to find a cure for the mutations, while struggling with the ethical questions about whether the mutations are a good thing. Right? Well, maybe. Or maybe not. Sometimes, the best main character is not someone who is a vital cog in the plot, but someone who has their own agenda.
You might have to try writing the story a few different ways, with different characters, before you find the right person or people to carry your tale forward. Often, it's about finding the person whose responses to the situation are the most unpredictable, colorful and organic.
Like, maybe your ideal main character isn't the captain of the ship, but rather a young girl who doesn't know yet if she's one of the mutants because nobody will let her see her own genetic data until she's old enough. Or maybe it's one of the young mutants, who feels like he's become better and stronger than ordinary humans and he's willing to live with the drawbacks.
A character who has a lot of passion, that is at least somewhat separate from the general concerns that every single character is coping with, is worth his or her weight in gold. People who are trying to figure out their own identities are often (maybe usually) more interesting than people who already know who they are. People who have something personal at stake also generate more sparks than people who are just concerned about the general welfare. Etc. etc. etc.
Also, your protagonist is usually the one who makes choices that drive the story. If all the interesting or defining choices are being made by someone other than your protagonist, who's just reacting to stuff... then you probably chose the wrong protagonist.
How does your main character's agenda run diagonal to your plot?
Again, it's not a requirement — but this is often a good question to ask. You have your over-arching plot, which is pushing in one particular direction, and then there's whatever your main character wants. Your protagonist's goal or desire doesn't have to be at cross-purposes to the plot, but it can run at a diagonal or almost parallel.
What does your main character have at stake, that is unique and personal? Every time the plot pushes your protagonists in a particular direction, maybe it's pushing them away from their goal, and that's where a lot of storytelling energy comes from. Instead of just being pushed along by the plot, like a log in a stream, they are swimming as hard as they can to go in their own direction.
Maybe your heroes don't have time to care about this mutant uprising, because they have to save their best friend who's been convicted of a capital crime and is about to be airlocked. They have to get to the airlock in time to save their friend, but meanwhile these stupid mutants are rioting and tearing the ship apart, and getting in their way. Fucking mutants.
Method acting, or maybe that other kind
Sometimes it seems like a lot of writers are failed actors. In any case, really good writing often has a little bit of acting in it. You have to do a bit of method acting, or maybe that other kind where you just project the emotions somehow. Whatever that kind is called.
Often, it seems like the best stories are personal in some way — not that they're necessarily about a thing that happened to you in real life, but that they are touching on something that's emotional and significant to you personally. Sometimes it's helpful to try and think about what that might be, and sometimes you don't want to delve too much into your personal shit, because you'll do too good a job of figuring it out and the urgency will go out of the story.
But at the very least, you may need to think about this story means to you, personally — and what stuff in it is most thrilling or joyful or upsetting for you. What's the part of the story that makes you cringe, or gets at your deepest fears or hopes? When you sit on the toilet and think about the story, without having it in front of you, what's the part that sticks in your mind, or what's the weird conversation you're imagining in your head between the characters? Sometimes, you just get one line of dialogue popping into your brain in the middle of the night, and that becomes a whole scene, which becomes the whole deal. One great line of dialogue, sometimes all you need to get your teeth sunk in.
What is the theme of the story? That's not necessarily the same thing as the main idea — the story might be about mutants on a spaceship, but it might be about survival, or loyalty, or friendship, or suicide. You don't need to decide "this is the theme of my story," but it can be helpful to think about what that might be, and how a particular theme might affect your characters and key moments.
Part of this has to do with that thing of writing a sincere first draft. You need to be truthful to the ideal version of the story in the back of your head, which you might not even be able to see fully right now, but it's in there somewhere. There's some reason you needed to write this story, beyond "wow, cool idea, bro." You have lots of cool ideas, but you chose to turn this one into a story. Once you know why you wanted to turn this idea, in particular, into a story, you're a lot closer to actually making it happen.
So there's a checklist of sorts, which might be helpful: make sure your idea is fleshed out into a series of events, which include some twists and some rising action. Make sure you're picking a main character who brings some juiceboxes to the party. And ensure they have their own agenda. And try to hang onto why this particular idea grabbed hold of you.
And then you'll be in business!