Coming up with a story + Pacing
For some, a random idea can be thought up on the spot, and then fleshed out into hundreds of chapters. For others, meticulous brainstorming may be required to get that perfect novel idea.
For those who can quickly come up with a satisfactory idea, good for you! Just make sure to keep your writing consistent.
For those who can't, don't worry, as long as you set your parameters you'll get there eventually!
Firstly, you need to define what kind of story it is you want to tell. Decide your genre, think of things that interest you, or stories you've always wanted to see get told. Even jotting down segments of random short stories you're making up on the spot could help you decide.
You don't need to fret, some people spend weeks deciding what their story should be, only to end up losing interest, and others have fun creating thousands of chapters from a novel they made up on the spot. The amount of time you've spent thinking of a story idea will not necessarily translate into how much you enjoy writing it, or how good it will be. So just relax and take your time narrowing down what it is you want!
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Pacing:
Characters-
When it comes to pacing characters, do it logically. That's really all there is to it, character pacing is entirely tied to the pacing of the plot, which is tied to the pacing of your writing.
Make your characters grow with the plot in a way that is reasonable! If your character is a normal high-school girl, and she accidentally kills someone, don't make her shrug it off unless you're writing a story tonally appropriate for jumping right into psychopath character levels.
If your genre is slice of life, and not horror, consider how a normal girl would react to killing someone, the consequences, guilt, shame, and so on. Think of how her character would grow from this, don't just treat it as a plot moment. And while we're at it, try and keep the progression leading up to the killing, and the consequences of her killing (you guessed it) logical! If you're going to have her magically get away with it, flesh out exactly how she gets away with it, don't just make it happen.
And also think about whether her killing someone and dealing with that guilt is tonally appropriate for the genre or type of story you're writing in the first place. If it isn't, maybe consider not doing it!
Think of your character's personality, their goals, how flexible their limits are, and compare all of this to what's just happened in the plot, a lot of good character writing and pacing is just 'walking in their shoes' and using your head.
Plot-
The progression of the plot flows on two things, the logic of what's happening, and your physical writing.
You need to consider how fast or slow your story has been moving, how long you've been sticking to a single segment, whether what you're doing next would take a long time to complete or not.
How important a specific area of the plot is can determine how you pace it. If it's something minor with little impact on the overall story, characters, world-building, etc. then just don't waste a lot of time on it. Readers can get impatient, especially when what you're spending 5 chapters on can be done in 2 without losing a single detail.
I'm against the belief that padding your story is usually okay (padding is basically filler, to make the story longer). The plot should always be developing the story, world, or characters in some way, otherwise, why write it? And if it only does a tiny bit of this and isn't extremely immersive or well-written, why make it long?
As for exposition, do not overload readers with information, such as several names or acronyms at once. Unveil information as it becomes relevant, not at your pre-decided world-building whims. Also avoid forcing a concept or moment into the plot simply because you liked the sound of it, start off slow introducing and developing the set-up, use patience, and allow your idea to unfurl naturally.
Side note, avoid having characters spew out expositionary dialogue that makes no sense. If the character is explaining already known information that no one in the story asked the character for, you're just wasting time and making your characters sound like plot robots.
A good example is a teen with latent talent as hunter in a world where these hunters are basically super powerful celebrities, and the doctor suggests the teen go awaken as one, but then suddenly goes off on a tangent about who the high ranking hunters are and how the government would fawn over them whenever they show up, even though all of this is clearly known by both characters and is irrelevant to the original topic at hand.
This is bad writing, a complete waste of lines, and it makes the character feel like an NPC.
Writing-
How long you take to describe something depends on its importance, I will keep this short.
Character enters a new fantastical place for the first time? Go knock yourself out with long beautiful, detailed descriptions if you want to, just don't make it take up an unreasonable amount of chapter.
Character enters their suburban home for the first time from the reader's POV? People's imaginations can handle a suburban home or an apartment just fine, you barely need to describe it if at all.
With normal mundane things, you hardly need anything greater than a short vague description. If there is a specific detail that needs paying attention to, either for the later plot, or for immersion, put in some extra words to make it stand out.
The reader's imagination can fill in a lot, so don't waste their time on what railings some random guy's house has, unless the said house is supremely important to the plot and the description of the rails are there to help paint a picture.
Describing the scene is not typically something that has to be done every time either, if your character goes through a field, there's not too much you need to describe outside of the atmosphere or some general observations. Your readers know what a field looks like.
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Something else that helps writing flow is avoiding long-winded sentences, paragraphs with no good spacing, and repetitive word usage.
Using the same word too many times in one area and make the writing seem amateurish, to give an example, "The other boys were going to attack but the leader stood up gesturing to not attack."
The problem is how the over-use of "attack," and the complete disregard for stops (commas or periods) dries up the emotion in the text and messes with the pacing, when text feels dry it can soak up a lot of enjoyment and immersion and make the story itself seem to drag on. I'm sure we've all experienced this.
If I had to rewrite that sentence, I would probably say "The other boys were readying to attack, when the leader stood up and ordered them to halt." Using a wider selection of words, more stops, and less repetition makes the line a lot juicier. Though that could just be my ego talking.
I highly recommend making yourself conscious of any words you tend to overuse and attempting to avoid repeating them too often. Use a thesaurus or the like!
Another thing to avoid is stating redundant information, if you've already had your exposition explain something, you don't need to state it again.
Of course (for example), it's fine to describe the processes of certain abilities whenever they get used, to help paint a picture, but actually describing the ability and its function multiple times can clog up the story. Stick to descriptive wording rather than descriptions, this goes for most information brought into a story.