Hi There
To step back a second, not all blog posts or pieces of content need a focus keyword. It depends on the goals of your content, and how you are trying to get it traffic. It looks like a lot of your content may be best suited to get traffic from social media etc. That's great, and in that case just write your article and don't worry about the focus keyword too much because it may rank for stuff by happenstance, but your main traffic sources won't be SEO.
That said, sometimes you may create content because you specifically see an opportunity to maybe rank for something where there is a "gap" in existing content in Google's search results. For example, you may notice a lack of great results for something like "pembroke welsh vs cardigan corgi" - and as a goal, you may set out to create the best piece of content on that topic to rank for it, and SEO is your primary goal. In that case, you'd enter "pembroke welsh vs cardigan corgi" into Yoast, and it will tell you how well your article is optimized for that based upon simple best practices.
So in other words - focus keyword is great when you know your target keyword ahead of time and are writing an article based off of that. If you're just writing an article for non-SEO purposes, maybe you'd refine a target keyword after the fact, but it's not a huge concern.
Choosing your focus keyword also has to do with just knowing good keyword research in general. There's two main factors to think about. Search volume and competition.
- Search volume - how many people look for the keyword per month. For blog posts it's best to stick in the 10-1,000 searches per month range.
- Competition - there's no single competition metric (Google does not provide one) but you can judge competition by experience of looking at search results or using a tool like Moz's keyword difficulty - which will give you a number from 1-100.
The trick of course is to try to find the highest volume but lowest difficulty, and that's a great beginning point for choosing focus keywords.