This is a true story.
A person, using a specialised platform for creating a course website was asking a question about adding keywords to the site. They weren't finding a place for their keywords. Apparently they were looking for some sort of a keywords field and since it wasn't there, they were confused. Naturally, they were worried about their search engine optimisation.
I get where they come from. With all the misleading, overly complicated and contradictory information about SEO and keywords out there, it all can be perplexing. The messy history of keywords doesn't make it any easier.
What's the messy history of keywords?
In the early days of search engines, they weren't very good at finding keywords within content. The solution to this was to add keywords to a special meta tag included in the source code of the site. This meta tag wasn't visible on the site, but search engines could find it and use it for ranking your site.
The problem with the meta keywords was that you could add anything there and pretty much dominate search results. Many started to take advantage of this, and stuff irrelevant yet popular keywords in the meta tag in order to get more visitors to their sites. Since you could add anything there, you could get your website to be found in pretty much any search.
As a matter of a fact, the whole keyword ranking thing was a bit of a mess for a long time. Besides messing with the keywords meta tag, people also would add other hidden text on their sites, listing keywords they wanted to rank with. However, this is a bad method, and may even cause your site being penalised by search engines.
As the search engines noticed the keywords meta tag was easy to manipulate, they started to phase it out. In 2009 Google confirmed (video) they don't use keyword meta tag in search rankings.
Some search engines, other than Google, may use meta keywords, or your categories and tags for tagging your content. This doesn't improve the actual search ranking, but may or may not help the people using the search engine understand the relevancy of the content. This isn't a very common practice anymore, so you don't have to get too involved in it.
Since the days of depending on the keyword meta tag, search engines have learned to see websites the same way as any person visiting your site would. What you see (when logged out from your site admin) is what the search engines see.
Many SEO experts believe keyword meta tag is completely redundant and you have no need for it anymore.
These days you still can see keyword field in some SEO tools. It may seem confusing, but it's usually used only to try and check if the keyword is really used in the content. Inputting the keyword in this field doesn't improve your search ranking. It's only for testing and evaluating purposes.
So, where do I add the keywords in my content?
Simply and shortly, you add them into your page content.
These days search engines can read the content of your website's pages and blog posts to find the keywords. Not only they can find the actual words in the content, but they can also find similarities and relations. For example, if your content is about search engine optimisation, it may rank in searches for the acronym SEO, even if you don't include it as is in the content.
Now, often people think that the content only means the unique content of the page, but it also includes header, sidebar and footer content. Basically any content you can see on the web page is that page's content. That said, footer and other such recurring content may have less affect in your search ranking than the unique content of the site.
Content in this case also includes the heading and subheadings of your content. Some speculate headings, including subheadings, have a greater impact in search ranking. Search engines do recommend using subheadings in your content, but they don't directly say they would have any impact in search ranking.
Another speculation is that content and keywords higher up on the page might have more impact in search ranking. This is conjecture, because search engines don't reveal publicly most of their ranking factors.
Don't forget to add keywords in your image file names and alt texts (alternative text). It depends on your platform or system if you can easily or at all add alt texts to your images. They may have impact in search ranking, or at least in image searches. Search engines are still pretty bad at trying to understand an image without the alt texts, so the texts are important in describing the image to them.
You can also add your keywords in your page's meta description, if that's available in the platform or system you are using. The meta description tag does not affect search ranking, but it may be shown as the content snippet in search results. As Google's official guide says, it's possible that a search engine will decide to show another snippet from the page content depending on the search. You cannot really make search engines to show your content in a certain way, but you can give suggestions.
Focus on the content and you are more likely to see results.
How do I add the keywords in my content?
Add the keywords naturally within your content. Whatever you do, don't just add the keywords as a list. If search engines get an idea that you might be trying to game searches and are keyword stuffing, your content will get penalised.
Keyword stuffing means listing irrelevant keywords in your content, or repeating a keyword in an unnatural way in order to improve your search visibility. Repeating a keyword to make the content seem more relevant may seem enticing. This can have an opposite effect, and make the content appear spammy. Especially very short content with repeating keywords is a huge no.
Always make your content easy to understand, and add the keywords only if they naturally fit in it. A keyword that doesn't fit naturally in your content should not be there or at least should not be repeated too much. Oh, and don't try to use too many keywords in your content. Search engines like specialising and niches.
To make a keyword appear as such, it usually needs to be repeated at least a few times and there must be other related keywords in the content. Just don't repeat it too much and in an unnatural way.
The best practice is to concentrate on making your content relevant and readable for people. When the content is relevant and readable for people, the search engines will see it that way too.
Is there anything else I need to know about keywords?
Even if you add the keywords naturally in your content, it doesn't mean your content will be automatically ranked or ranked high in searches. Some keywords are more common than others, and there may be lots of competition.
This is why lots of SEO advice suggests using something known as long tail keywords. These are keywords with less searches and thus less competition. Basically I recommend to try more specialised keywords and not only target the most common ones.
Keywords are also only one of hundreds of search ranking factors. The keywords and content in general are considered to be the most important factor. Another important factor is getting links to point from other high ranking and relevant content to your content.
Search engines are constantly modifying their algorithms. What works today may not work after the next update. At the moment Google, and perhaps other search engines too, appear to be very concerned about so called thin content and are trying to concentrate on content quality with mixed results.
Presently search engines can already understand relationships between keywords and their relevancy to the rest of the content. It is likely they will "learn" to be even more vigilante about the relevancy of keywords and the content in general.
In short, keywords are important, but not by far the only factor in search engine optimisation. They need to be added naturally within the page content, including the headings. They must be relevant to the content and, while they need to be repeated, too much repetition is bad. Balance is the key.