When Ben Franklin wanted to learn to write well, there weren’t a lot of options. There weren’t any writing courses and even if there were, Ben was working long hours as an apprentice printer. So he did the logical thing.
Franklin identified excellent writers and analyzed what they did. He chose the writing of Joseph Addison and Richard Steele. First, Franklin would read one of their essays. Then he would write down the substance, in order. A week or so later, he would try to re-write the essay from his notes and then compare it with the original.
You can do the same thing. Identify a writer you admire. Read their work. Try to reproduce it. Repeat until you get it right.
You can also study other writers who are masters of a specific technique that you want to learn. Study great business writers like Peter Drucker, Tom Peters, or Jim Collins. Study great fiction writers like Tom Clancy, Elmore Leonard, or Louis L’Amour.
Let me warn you, this is not easy. It takes discipline and it takes time. But it’s a way to make your writing dramatically better.