1) Get an idea that's worth developing. Honestly, this is the easy part. If anyone out there has the notion of "I've got a great idea! I'll give it to an author, s/he can write the book, and we'll split the proceeds," please don't bother; most authors have more ideas than they can ever write up, and most also know better than to commit themselves to a deal that leaves them doing 99.9 percent of the work for what will probably be very little return. (If you haven't read my August 17, 2023, post, "So You Want to Write a Book," I'd suggest doing so to get some grasp of the work involved and the likely returns.) Besides, if you don't feel passionate enough about your idea to put the work of writing in, why should someone else?
2) Research, research, research. If you are writing nonfiction, this is an absolute necessity. Remember, you're trying to sell to an audience with an interest in your topic, and that audience is going to include some people who aren't going to be fooled by blowing smoke. If you count on AI summaries to do your homework, you deserve what you get. Fiction writers also do more research than you'd think; it's part of creating believable fictional worlds in which suspension of disbelief can operate.
3) Create a manuscript and/or a book proposal. This is where the road forks between fiction and nonfiction writers. Generally, fiction publishers want to see a complete draft accompanying your cover letter and proposal. Nonfiction publishers vary more; some will be content with an outline and synopsis, while others will want to see sample chapters. Find out what the requirements are for the publishing house you are targeting and follow them religiously, especially if you have no prior history of working with that house.
4) Prepare to be hit with a lot of advertising for publishing opportunities once you start marketing your book proposal/manuscript. Most of these will be for self-publishing or vanity presses; some will be outright scams. If you do want to go the self-publishing route, do your due diligence about researching how best to go about it. (While my crystal ball may be no better than anyone else's, I think it not improbable that the current flood of AI-generated books hitting the market may actually lead to a resurgence of more traditional publishing in some form as readers look for books that have actually been written by humans and have passed some sort of quality control prior to going to market.)
4) You've been accepted and you've signed a contract! Congratulations! But now, the grunt-work really begins. If you haven't already come up with and submitted a complete manuscript (including any illustrations called for in the contract), you will have a deadline for doing so. If you have, your next step will be copy editing and revisions. The exact stages will depend on the house you're going through. At my current publisher, the University of Kentucky Press, two or three readers will go over a manuscript first, giving feedback and suggesting areas for improvement. After any revisions have been made, the next step is copy editing. That means a copy editor goes over your work line by line, looking for everything from punctuation errors to unclear writing, and you will need to review and respond to every suggested change (though you need not incorporate every change if you feel you have good reason for standing pat; it's your manuscript, after all). If any major changes need to be made that haven't already been taken care of, this is your last opportunity to make them.
5) Review of the proofs is the last major stage before publication. At this point, you will be looking at your manuscript pretty much as it is going to appear when printed. You should read it over carefully and make sure any typos or mistakes haven't managed to sneak through the earlier editing, but suggesting major revisions now is a good way to have your editor frothing at the mouth. Even if the revisions are accepted as necessary, they will result in the entire manuscript having to be set up anew for printing and may well delay the release date, which will not make the publisher happy.
6) If this is a nonfiction work, you will probably be asked to create an index for your book, making it easier for readers to locate specific information. (If this is going to be done by the publishing house, count yourself lucky; it isn't a difficult task, but it is a bit tedious.) The reason this comes so late in the publication process is that you'll need to be able to link your topics to page numbers as they will appear in the printed book, and those numbers won't be available until the proofs are created. Get the index done on time in order to keep from delaying publication.
7) Your book is born! You will probably receive the author copies specified by your contract two or three weeks before the actual release date, but at this point, the writing work is done ... until the next book.