The "short" path to become a pro writer:
1) Create a website with a blog
You need your own domain name to drive access to your brand through SEO and keep full control of your content.
2) Get into a habit of daily writing, publishing, and sharing
Write anywhere, publish on your blog, and share everywhere (without copy/pasting, adapt to the channel). Set canonicals URLs to your original blog post whenever you can. Don't give up on a channel after a few days, but stick to what works best to drive traffic to you.
3) Interview people in your niche to build trust and domain expertise
Interviewing is a great way to grow your network. Done well, it's a win-win situation for you and the interviewee: you will both acquire a new perspective on your work (great questions, just like great answers, trigger self-reflection) and benefit from each other's network.
4) Get into a daily reading habit and use it to fuel your ideation process
You can't write well if you don't read. In addition, a piece of content without references isn't trustworthy and will appear amateurish. Do your research, and share it generously.
5) Organize your writings by topic
Start curating your content early on. When you have thousands of short posts, having an overview of what you wrote helps finding opportunities.
6) Keep going until you reach at least 30,000 published words for a given topic
30,000 words can be considered a minimum length for a non-fiction book manuscript. It's short, but it's better to offer to-the-point content than to ramble on and on.
7) Curate and rewrite your blog posts into a book
Books are the most straightforward way to make a living from writing. People still pay for books, and there is no need to explain why as long as you are clear about what's in it for them.
On the contrary, monetizing blogs is much harder. You'd need qualified and/or ginormous traffic, which isn't easily predictable. A book approach is more straightforward: solve a problem with your words, and sell the solution.
8) In parallel, provide content services in your niche through your website
Package your offering into productized services and use marketplaces like Fiverr or Gumroad to distribute them. Every company needs content marketing these days, so there is still room for talented and/or niche writers with a specific expertise.