I'll be the first to admit that choosing keywords is hard. And a pain in the buns. And it's kind of like an Escher painting; the more you think about it, the harder it gets. Most of the time, I find blogger-level SEO to be pretty straightforward, but choosing keywords is the one area everyone I know struggles in, even SEO "experts." But it doesn't have to be this way.
So let's back-back-back it up for a second and get back to the keyword basics.
First, when we talk about "keywords," we really mean "keyphrases." And finding the proper keyphrase is sort of like hitting a homerun, you've got to hit the sweet spot on the bat. When we're talking SEO, the keyphrase "sweet spot" is a 2-4 word phrase that is broad enough that someone, somewhere out there is actually searching for it, but not so broad that everyone in the world is searching for it. When lots of people are searching for a keyword/keyphrase, here is generally a lot of competition to "rank" in Google and Bing search results. Lots of competition is bad. Moderate-to-low competition is good.
Let's look at an example:
Say I'm writing a blog post about the Chicago Cubs Spring Training schedule. I don't want my keywords to be "Chicago Cubs," because it's just way too broad. I'm going to be competing with the Chicago Cubs themselves, all the ticket brokers that sell tickets for the "Chicago Cubs," every media outlet that cover the "Chicago Cubs," and God knows who else to rank on the first page of Google or Bing.
But "Chicago Cubs Spring Training," or even "Chicago Cubs Spring Training Schedule" is a much narrower search and it's much more likely that I could rank for that term. So I should use "Chicago Cubs Spring Training schedule" as my keyphrase. Once my keyphrase is chosen, I need to remember to do all the things with it that we talked about in our last SEO post (front-load it into my title, use it in my first paragraph, etc).
Likewise, the term "scarves" is way too broad to use a keyphrase. But "how to tie scarf" or "ways to tie a scarf" is much more narrow and therefore much more likely to rank highly.
One of the best ways to figure out if you can compete for a search term (outside of using Google's Adwords Tool, which is another blog post altogether), is to Google your chosen keywords/keyphrase yourself, and check out the first page of search rankings. If you think you can bust onto the first page (i.e., the first page isn't completely taken up by monster sites like CNN, Amazon, ESPN, Mashable, Glamour, etc.), it's probably worth a shot.
And remember, Yoast is a tool to help you check your work. not a separate SEO component in and of itself. If you know you've done everything write, ignore Yoast and hit the "publish" button.