That link is a great source of concise, clear, practical advice. I did an enormous amount of research on SEO and so on when I got serious about writing. I never came across better advice.
The most important thing is that you engage with others on whatever platform you choose.
Absolutely right. Blogs and other forms of "social media presence" will not sell your books. Not even massive PR campaigns sell many books. Publishers only do big campaigns when they feel the book will sell itself when readers read a bit of the book. And most of us only do THAT when some readers are enthusiastic enough about the book to encourage their friends and acquaintance to read the book.
A social media presence is not about selling your books. It's about establishing a relationship with other authors and with your readers. It helps you feel less alone, helps you discover all the things in common that artists like yourself share. The problems and the triumphs. It also helps you maintain rapport with your readers.
Absolute Write is a big help in doing all that, which is why I've been with it for years, and contribute a bit of money whenever I feel my finances allow it.
Should I wait until my book is published to create a blog?
It's up to you what feels most comfortable. Your job is writing books, not writing blog posts.
IF you decide to do it, start early, BEFORE you need it. It will eventually be the one way you have complete control of your "brand" - the face you present to the world. It takes time to learn the art and tech of creating a blog.
START SMALL, TAKE BABY STEPS to add stuff and redesign your site to establish your unique brand. STAY SMALL as you can. A simple beautifully elegant, easy to get around in, site will be much more impressive than a jumbled kitchen sink kind of site.
In the "static," rarely changing part, put a brief bio and maybe background info on the universes you write in, such as genealogies and timelines and so on - stuff you don't want to wedge into your stories but which your readers might like to know.
In the blog part of your site, I suggest you put in whatever you think your fans might enjoy sharing. BRIEF posts about new books, movies, TV shows, fan conferences - if you're very engaged in something such as raising horses or dogs, historical eras, auto racing, space exploration likely ever week or two you'll come across something you would like to share.