I was also faced with this question couple of years back.
Firstly, You can check out this post on reddit, it has helped me a lot;
https://www.reddit.com/r/Entreprene...to_write_innovative_and_competitive_longform/
Also, I can't recommend the book 'Everybody Writes by Ann Handley', enough. Buy it, worship it, sleep with it, and take a shit with it.
Secondly, read everyday. Fiction, non-fiction, sci-fi, blogs, it doesn't matter but read everyday. And don't just sprint through aimlessly, that's
the mistake I made. Examine the sentences, flow, structure, vocab. Notice why the writer wrote a sentence like:
"He pressed his hands against the edge of the table, and looked at me with gleaming eyes."
and not,
"He put his hand on the table and looked at me."
There's a famous quote from Anton Chekhov that has stuck with me.
He writes,
"Don't tell me the moon is shining; show me the glint of light on broken glass."
This is what I'm talking about.
I know this advice feels more relevant to creative writing but for me, analyzing what I'm reading has helped me more than
any guide or course online.
Also, write everyday. People recommend starting with 300 words a day, but I started with 50 a day.
Lastly, you can take some courses on Udemy, Lynda, or other similar sites. Personally, I've gone through two course from Shani Raja named
'Writing with Flair' and 'Ninja Writing'. I recommend them only if you're complete newbie and need a basic intro/framework to writing.
But beware, they're very boring and dull.