We could begin here: writing is a series of decisions. (I ﬁrst heard this from Mary Robinette Kowal.)
I talk here about ﬁction writing but it could apply to non-ﬁction as well.
There are macro decisions—about format, about plot, about character, about theme—and micro decisions—about syntax and sentence length, word choice and punctuation.
This is why the initial blank page is so scary: all the decisions, both macro and micro, are still there ahead of you waiting to be made.
And this is why, when you near the end of a ﬁrst draft, the pace of your writing seems to rocket ahead: the large majority of your macro decisions have been made, and you’re able to round out the consequences of your decisions.
Becoming a better writer then is about learning to make better decisions.
Like any skill, learning to write is fractal. That is, as you learn to write, it will initially be learning what are the wrong macro decisions (hackneyed characters, stories that have been done before) and wrong micro decisions (incorrect grammar, cliched word choice).
Once you do this, it becomes easier but also harder. Because as wrong choices are eliminated, more choices become available to you: new decisions that you weren’t aware of before, subtleties between decisions that you didn’t realise were all that different.