I DO NOT ADVERTISE. I just write book after book, a dozen so far in ten years. Every once in a while I'll write a shorter story that "fills in the cracks" in the universe I've created.
I have a website which is a central location that I have complete control over & acts as a conduit to and from my readers.
I post notices on Facebook & Twitter but most of my activities there are social, not marketing. Over the years the amount of my followers has grown fairly hefty.
On Amazon I have an Author page that lists all of my books. Readers can sign up to get a notice when I put up a new work. Be sure to take Amazon up on establishing one for yourself. It's very useful. Here is an example of what one looks like.
Over the years I've continued to gain readers. I've got to the point that every month I pull in so much money that I need a tax accountant. Self-publishers have a number of tax breaks. Don't do your own returns until you learn what they are and can do your returns intelligently.
Bascially, your biggest advertising are your books, not anything else. Every book you put up advertises all the other books. So much so that my very first book years ago is still selling.