However from the article: "First, you need to select a theme, usually related to your professional goals, and then set up a newsletter. Examples of themes can range from database optimization to product marketing".
As a rough hewn cognitive equivalent of a Kinder Surprise, I find this on a personal level utterly impossible. I'm sure everyone has their own style of writing, but I don't start with a concept and work from there. Instead, I furiously flop my lobes around like a neurological snow globe and then calmly wait for the dendritic snow to settle. So my blog starts with an article on how to build a better IDE, then goes into the construction of an infinite wish generator, followed by a paper on the biological constraints of free will, and bla bla bla bla.
I'm aware that this makes my blog virtually immune to even the concept of a target market except perhaps for the most extreme spectrum of ADHD, but I'm resigned to it.
I think that building on an established domain is fantastic and is probably the most effective way to build a following, so I'm always curious how others manage to retain that level of focus.