In a word, short stories.
Write a bunch of short stories. One or two a week if possible. Keep that up for a year or two, tapering off at the end to transition into novels. But keep writing short stories even after novels have become the main focus.
Make a serious effort while writing short stories to master both the craft and the art of storytelling. View it as an apprenticeship period. Experiment. Try out new things. Join a writing group, preferably of experienced professional writers, and have them rip your stories apart. Soak up as much constructive feedback as possible, and apply it to the next story.
At the same time, don’t spend so much time reworking old stories that you aren’t producing new ones. Learn how to keep a rigorous production schedule. If a story is totally broken, toss it out! Get to the point where you can hit 2k words consistently every day, and knock out a story at least every couple of weeks or so.
In a word, learn how to be prolific.
Experiment with standalones, but also build a couple of universes with recurring characters. Write a few series, both sequential and non-sequential. Focus especially on the non-sequential series, though—the ones where any story can be an entry point. Learn how to find the sweet spot between writing a satisfying ending and leaving a hook for the next one. That sweet spot is different for every genre.
Submit every story you write to the traditional short story markets. Start with the highest paying markets and work your way down. Pay close attention to average response times on sites like the Grinder and don’t submit to any market with an average response time of more than 30 days, no matter how high the pay rate. The goal is to get each story through all of the pro- and semi-pro markets in about a year. If a market can’t get back to you in a timely fashion, it’s not worth your time. Ideally, you want to be receiving multiple rejections every day.
Once you’ve got about twenty or so stories that have come off of submission, start self-publishing.
Use the first couple of stories to learn how the process works. Figure out how to format, do cover work, and write up all the metadata on your own, then do all you can to streamline that process until it becomes automatic. You can outsource some of the more difficult stuff, but learn to do as much as you can on your own. Don’t spend more than about $50 per story to publish it, preferably more like $30.
Once you’ve got a process down, set a rigorous release schedule of 2 stories per month. Keep to that schedule religiously. Don’t worry too much how the stories are selling: they probably won’t sell well until you’ve got a couple dozen or so out. Just focus on getting them out.
Keep an email list, with links to subscribe in the front and back of all your books. Build that list as much as you can. Most of your early marketing efforts should go to building that list, and cultivating a relationship with the people on it. Don’t rely on Facebook, because you don’t own that site and can’t control it. Same with any other social media. Do all you can to bring your readers to a place you control.
Start blogging. Build relationships with other bloggers. Strive to post something new every day. Make it the kind of site that your readers will want to come to. Be sure to have pages for all of your books, as well as a series page that lists every story in every series, in chronological and written order (side note: I really need to write up a series page).
Experiment with free pulsing and price pulsing. Experiment with price points. Experiment with bundles. Experiment with everything.
ORGANIZE YOUR DATA. Ohmygosh. You’re going to be drowning in data after just a few months. Keep all of your sales reports, and compile all that into spreadsheets showing how many sales you got of each title each month, how much you earned from each title each month, etc. Data, data, data! Learn how to thrive with data!
Write a formal business plan, and update it constantly as you go. Write down all the strategies that work, as well as the ones that don’t. Write down all the strategies you want to try out. In case it wasn’t obvious, write down your release schedule. Write down your to do list, organized by urgent / not urgent and important / not important quadrants. Write down everything. WRITE IT DOWN.
Eventually, you’ll get to the point where you’re releasing bundles alongside or even in place of your short stories. Don’t unpublish anything. Maybe update the covers, if you decide your early ones are really really bad. But don’t worry about it too much. Just focus on being as prolific as possible.
As long as you keep moving, you’re going to get somewhere. So always keep moving. Even when you have a disappointing sales month, or a spat of bad reviews, or whatever, just keep moving. Even if you’re moving in the wrong direction, that’s better than not moving at all.
At some point, you’re going to start to see some success. You may even have a breakthrough. At that point, you can start moving on to novels. Hopefully you’ve written a couple of them by now. Your first one is probably utter crap, so toss it out and focus on the good ones.
Hopefully, you’ve written it in the same universe as a bunch of your short stories. That will make the marketing easier, but its not strictly necessary so don’t worry about it too much if you haven’t. Also don’t worry too much if the novel isn’t in a series of its own. It’s better if it is, but standalones have their place too.
Try to write in trilogies, or to write standalones that can easily be turned into trilogies. The first book should stand on its own, the second should end on a low note and hook into the third book, and the third book should blow the reader’s mind away. Brandon Sanderson’s Mistborn series is a great example of this.
If your career hasn’t taken off by now, you aren’t experimenting enough. That, or you’re cutting too many corners. One way or another, you’re going to have to put in the work.
That’s pretty much it. Have fun!