Advice given here would depend on the definition of 'what is a good writer?' Some people will think a good writer is someone who is successful at their trade and has a string of bestsellers to their name. Other people will think a good writer doesn't care about book sales and will be most concerned with the quality of their words. My feeling is that there is a happy medium and often the winners of the best known literary awards fulfill both criteria. My personal definition of a good writer is someone who can write for any audience and enthrall them with a compelling story, simply told.
Being a good writer is more than writing well. Knowing how the publishing industry works and what audiences want to read is also important. However, the following advice concentrates on some things that will help an enthusiastic beginner become a professional author. If a writer produces good work, the other skills and experience will come in time.
The following outlines some habits that writers should get into. Acquiring these habits won't on their own create a good writer. However, a talented writer who follows this advice won't be wasting their time. Each item on the list will improve the writer's understanding of their craft and improve the skills they need to elevate their work from average to excellent.
Read a lot.
It's not possible to dodge this one. A good writer should try to read plenty of the kind of thing they intend to write. They should also read books that will provide background facts and ideas. A good writer must also read outside their chosen genre and other genres that don't appeal to them. Only authors who have read widely will have the reading experience to know what makes a good book.
Read good books.
A good writer must read good books - classic authors, contemporary writers who are acclaimed for their prose style, winning titles of literary prizes. Bestsellers can be good writers but often a book will sell for reasons other than the quality of the writing. Bestseller doesn't always mean it's a well written book. However, writers who write stories that people want to read, and can do it with style and quality, do sell books.
Write something every day.
This is advice writers often hear, want to do but then fail to do. It really means write habitually and frequently. Being a good writer is as much about practice and momentum as it is about any of the other habits mentioned here. Writing everyday reduces the pressure to produce something a writer feels after a break in writing output. It also means that the writer is productive. Even if - for example - a script isn't very good at the time it was written, the rough draft provides the basis for something better later on. While it's a good habit to write everyday, it's not always necessary. Writers who work at their novel from nine in the morning to five in the evening, Monday to Friday can safely take the weekend off. Writers who only have the weekend to work, should also try to find a time during the week to write.
Keep a journal.
Keeping a journal will help a writer to write something every day. Journal writing helps the writer learn to summarise a situation they've experienced. Writing for an audience - who may be reading the journal long after the events being described - perhaps the author themselves - is good discipline. It makes the author consider the most pertinent facts and the parts that are missing that need to be explained and filled in.
Keep track of books read.
A writer should keep a note of the books they've read and jot down anything about them that seems unique or interesting. A short review allows the writer to consider the merits and flaws they've seen and so learn from the book author's mistake. Keeping a note of at least the title and author will help the reader to remember the books they have read without rereading them and a short synopsis will also allow them to remember what the book was about.
It is useful for a writer to read their own work aloud to hear how it sounds to their audience. Writers sometimes forget that grammar and punctuation are there to make the words 'sound right.' Reading prose aloud will show any flaws in the sentence structure and natural rhythm.
Study the rules of grammar, spelling and punctuation.
These rules have been been developed by generations of writers and academics all seeking to make their words as clear as possible to as many people as possible. However, not all the rules have equal weight. Understanding the rules allows the writer to play the game and know how to win the game by breaking them.
Read 'how-to' books on writing and writer's biographies.
Learning to be a good writer is a never ending journey. Reading as many sources of advice and experience as possible, is the best way to make that journey. Libraries are full of books on how to be a good writer and how to avoid the pitfalls of both writing and publishing.
Keep it simple.
A writer's job is to communicate an idea using the written word. Simple words and ideas are easier for the reader to understand.