So, I have a blog now. Do I just write?
Not exactly. You have to think about what you write. It needs to fit your theme and audience — if you’re writing to writers about writing, writing about politics will probably feel “off”. You can be an essayist in your blog and write about whatever crosses your mind, but it may be harder to convey a theme in your blog.  On the other hand, John Scalzi, a science fiction writer with a multi-book contract, writes about everything from writing to politics. His blog, fittingly, is called Whatever. If you’re as big as John Scalzi, you can write whatever.

How do I know what to write?
If you have a theme and an audience, you’re doing them no favor by just free-writing. This is your personal/professional presence on the Internet. If you’re a writer, it’s a chance to market yourself. You can compose directly on the screen, but compose. cut and paste to make the document flow. Proofread. Read it aloud to yourself like any other document you want to write to impress people. 

The blog should not be too long, in order to cater to today’s readers. I’m divided on how excerpts of novels and short stories should be presented in blogs, but I think that short excerpts are the best.

The 29th Annual Edition Guide to Literary Agents 2020 (Brewer, 2019) suggests that one should have a list of topics for six months or a year. This works great if one writes weekly; I write daily (which is insane, but so be it). I do have a list of weekly topics, but intersperse those with more personal observations about being a writer.

Other suggested tricks
The 29th Annual Edition Guide to Literary Agents 2020 (Brewer, 2019) has a chapter that presents several other reader- and SEO-friendly suggestions for blogs:

• Short paragraphs and headings, as you see here. This works well for informational blogs; not so much for narrative.
• An SEO tag in the title. I have a few that I use for my blogs, although I sometimes forget these because they’re new to me.