This is something that I have also wanted to do for a long time, but have not. I’m sure that there are other published authors that can give better advice, but mine would be to just start writing and see where it takes you. Sure, it’s not gonna be a bestseller or even very good unless you are a prodigy, but it will give you a feel for how it is done. Getting your story in the hands of others and listening to feedback can also be invaluable. If you have written a short story that you like and got good feedback on you can also submit it to websites that publish short stories. Just Google “East of the Web” and look at the short stories written by other people for inspiration.

1 Make the time to write and stick to it
2 Don’t be afraid to rewrite things that doesn’t work.
3 Read your dialog out loud and listen if it sounds natural
4 Don’t become discouraged if it is difficult, if it was easy everyone would have a bestseller.
5 Work on an outline first
6 Keep an eye on your pacing
7 Save the editing for later
8 Start short if you can and hone your craft
9 Get feedback
10 Read a lot

I found that now that it is lockdown for many places I have more time than ever to sit down and write. It also showed me that “not having time” which was one of my biggest excuses in the past actually doesn’t hold water. My biggest problem is actually myself, more specifically wanting to watch Netflix, read other books and generally procrastinate instead of actually sitting down and writing. The tips that Mariana posted above are actually really useful so I would suggest following them as they have helped for me.

My answer is based on the only of my own stories & audio-adaptations, which are appreciated at all:

1: Learn the so called author 101, the basics of writing, grammar, and WHY readability is mandatory.
2: Practice a bit to either form your free flow, or follow the academic formula of structured approach.
3: Learn from your ordeal, as in example the kinda writing you excel with can be more important than the newbie longing (maybe you loved novellas, but your readership only appreciates your flash-fiction length, or full novels instead.)
4: Do some copycat stuff, so you learn to fluff-up and rewrite stolen ideas into a legal version, but do not mistake such for real writing (any ghostwriter does more).
5: Find the mandatory balance between marketing, time for yourself, writing, and maintaining your readership.
6: Survive, enjoy, boast with all the bestsellers, which I certainly will never outmatch. ;-)
7: In the long run staying true to yourself (your writing style, genre, characters) will at minimum create a secondary readership. Learn to handle the money aspect AND the readers you care about. Repeat, what I wrote as 6: ;-)

Some tips from someone who has tried and failed many times to write a fiction story of my own so take them with a grain of salt.

You have to love your own story otherwise it will become a chore and you will give up.
Don’t feel like you need to spell out everything for readers. Keep them guessing
Some of the best writers have very simple and straightforward sentences. Don’t try to be Vladimir Nabokov.
Make writing part of your routine and don’t skip days.
Before starting set some milestones for yourself and then stick to them without fail.
Always use active voice
It’s fine to take a break when needed
Worry about editing later, that’s the job of a good editor and not a writer