I’m looking at this question specifically from the POV of newbie authors, though I suppose the general answers would apply to anything.
How do you improve your writing?
First, you need to read, a lot, in a lot of genres. Read books about writing, like “On Writing” by Stephen King, or “Eats Shoots and Leaves” by Lynne Truss. Honestly, many best-selling authors have written about their writing process. The more of these you read the more you find there is no right way to write. Also, read everything you can in the genre you write. If you want to write a memoir, read memoirs. Epic fantasy? Read George RR Martin, David Eddings, R.A. Salvitore, and others.
Second, you need to learn the basics of spelling, grammar, and punctuation in the language you plan to write and publish in. Yes, you will be hiring an editor to catch your mistakes. Editors are cheaper if you have fewer mistakes to catch. It’s your job to understand the basic mechanics of your chosen language. There are online courses you can take. I’m a native English speaker and I still took an online course in grammar.
Third, don’t expect everything you write to be good enough to publish, especially in the beginning. You need to practice. There are no short cuts on this. I talk about this more here.
Fourth, slow down and spend some time in the pre-writing phases, as I explain in this blog here. Experiment with outlines, world-building, and character building. If you get stuck, go back to these steps and get to know your characters and your world better.
Get feedback when you can. Writers groups and beta readers are great for this. Keep in mind that family and close friends don’t make great critiquers, even literary-minded ones. They may be unsure of how to give you criticism without hurting you. When you get feedback, look at it honestly and accept it as part of your growth.
I’m in taekwondo and I regularly have more senior belts point out where my form or technique needs work. My response is “thank you”. They are not telling me these things because I’m bad or wrong or useless or anything like that, but because they want me to improve. I want to improve too. That means we’re on the same team.
The same goes for your writing. Beta readers and editors are not criticizing you because your writing is bad, they do it because they want to make you the best writer you can be. Thank them for their help, look at their advice honestly, not defensively, and use it to improve yourself.
That’s it. Read lots. Learn about the mechanics of language. Write lots. Let go of your inner critic. Learn to take advice and criticism and use it to better yourself. Practice, practice, practice, and never give up.