1) Write a lot. At first, write without any intention of showing it to others, and without any judgement about how good or bad it is. Just write.
2) Sit down and analyze examples of good writing and what techniques/habits they employ.
3) At first, emulate the style of authors you admire. Your own style will emerge later.
4) Recognize that most of what results in a piece of good writing is good editing.
> examples of good writing
do you have any favorites?
In On Writing Stephen King talks about a mindset of "Write once with the door closed, and then again with the door open". I interpret this to mean what you say. Write for yourself first, then revise / edit / rewrite with the intention to share with others.
4) Recognize that most of what results in a piece of good writing is good editing.
Two of my favorite writers, Stephen King and Dean Koontz, have both made very similar points at times. Koontz famously said he writes his books one page at a time. That is, he writes about a page worth of material, then revises / edits until he believes that page is good, and only then does he move on. King also emphasizes that you don't start out writing prose that reads like the (published) prose of your favorite authors.
So yeah... just write, and then revise ruthlessly.
I'd actually recommend reading that book, even if your interest isn't fiction (and definitely if it is). There's some good stuff in there.