Those of you who became great writers through practice, in which ways did you do so? I want to become a good writer but I don’t know where to start.

Mostly I just wrote. A lot.

The other thing I did was re-read what I had written. I didn't have much of a choice, since pretty much all my stories are part of the same series. If I didn't want to contradict myself (and I really, really didn't; that's one of my biggest pet peeves when reading), I had to re-read my own material.

Re-read your shit often enough, and first you'll start to notice that it is, in fact, shit. But next you'll start to notice exactly why it's shit.

That's when you can start improving. Either you realize what you need to edit to fix your common mistakes, or you can stop making them in the first place because you know what to look out for.

You can also solicit feedback from friends, but it helps if they're also writers, or at least people who will be honest with you. “This is the best thing I've ever read” is not going to help you improve your craft.

But yeah. Write. And then write some more. Finish that book and start the next.

Once you've got about ten novel-length books under your belt, you should see a vast improvement from where you started.