Core keywords define what your site is about. They are a starting place for stemming off almost endless subtopics, they are your most valuable keywords, and they should be one of the major focuses of your site's navigation and architecture. Here's how to go about finding out what they are.
1. Start With Your Existing Products and Promotions
Collect a list of the phrases that you use to describe the products and promotions on your site. Specifically, we're talking about high level descriptions. That means avoid adjectives and other descriptors. You're looking for phrases like "samsung galaxy," not "best cheap samsung galaxy s7 outlet reviews."
2. Get Ideas From The AdWords Keyword Planner
Start plugging your phrases, one at a time, into the AdWords Keyword Planner "Get Ideas" tool:
Now start browsing through the first page of the "keyword ideas" tab for some standout phrases that match your products as well as a decent amount of traffic. The phrases you choose should not be tangential to your products; they should describe the product itself or indicate a desire to buy it. No "reviews," keywords, no "information seeking" keywords, etc., unless of course your products are reviews, ebooks, etc.
The amount of traffic considered "decent" is going to vary pretty heavily by industry. For a very, very rough estimate of "decent," multiply the "suggested bid" for the keyword by the amount of traffic, then divide by 100. The result is within the "ballpark" (hopefully within an order of magnitude) of the revenue you can expect from that keyword if you rank on page one. Ask yourself if this is worth monthly effort to maintain.
As an example, suppose we were selling Samsung Galaxy S3s. This is an example of some of the keywords we might pick:
Next jump from the "keyword ideas" tab to the "ad group ideas" tab. This organizes keywords into subtopics that are related to the phrase you inserted into the "get ideas" tool. Clicking on each one of these will give you a list of keywords to check through. Check through the ones that appear most relevant with a similar rationale as above.
Continuing our example from above, these are some of the lists we would take a look at:
As you discover promising keywords, place them into a spreadsheet with a "keyword" and a "traffic" column, as in this example:
3. Estimate Keyword Difficulty
Estimating keyword difficulty comes down to looking at what sites on the front page are doing to rank for the keyword. There's no perfect method of doing this, but the best place to start is by measuring the page authority of the pages on the front page of the search results, and finding the average.
If you want to do this for free, you can use Moz's Open Site Explorer. If you want to do it quicker, you can use the ahrefs.com toolbar, which will require a subscription to be useful. Bear in mind that Open Site Explorer and ahrefs.com use different methods of measuring page authority, and their numbers should never be compared directly.
Open Site Explorer:
Do a search for your keyword phrase in Google. Ignore any nonstandard search results such as Knowledge Graph answers, Google News, Images, etc. Personally, I choose to include video listings as normal search results. This is up to you.
Open the links and then copy the URLs, one at a time, into Open Site Explorer:
Then copy the Page Authority metrics to your spreadsheet. Create 10 columns to store page authorities for each of the links on the front page of Google, and create another column to calculate the average page authority. Find the Page Authority for every standard search result on the first page of Google and copy it into up to 10 of the columns. I emphasize front page of Google, not the top 10 listings, since many, often most, of the front pages have fewer than 10 standard listings these days.
Your spreadsheet should look like this for each row (the Moz page authorities here are made up):
Capturing the URL authorities using the ahrefs.com toolbar is a lot easier, but, as mentioned, you need to pay for ahrefs.com.
After signing up for ahrefs.com, get the toolbar here.
Make sure the toolbar is active:
Then run your search query, and copy the Page Authorities from the search results, skipping any non-standard search results:
Copy the Page Authorities into your spreadsheet and calculate the average like so:
4. Making A Judgement Call For Each Keyword
There's no perfect method of determining which keywords to target, but once you've identified a list of relevant keywords and assigned a difficulty based on Page Authority, you can begin to identify the feasibility of ranking for each keyword by comparing your average Page Authority for each keyword with the SEO metrics of pages with comparable Page Authority.
In other words, if your average Page Authority for a keyword were 10 in Moz, take a look at a few URLs with a Page Authority of about 10. Take a look at the number of linking domains in Moz, the types of links, they domain authority of the links, etc., and devise an action plan for obtaining comparable results. Likewise, take a look at the pages on your own site. Are any of them already at 10? Are they at 10 based solely on internal links or do they have external links to prop them up? Are any of your existing pages a good fit for the keyword you are trying to rank for, and do they already have a comparable Page Authority? If so, your best bet is to update that page.
A note on Moz Page Authority in particular: it tends to offer low Page Authorities for URLs that don't have any inbound external links, even if they are on a site with a high Domain Authority. This is an important thing to keep in mind that can skew your target Page Authorities lower that they might otherwise be. If, for example, you find a URL with a Page Authority of 1 but a Domain authority of 99, odds are you won't be outranking it unless you also get a link from a site with a Domain Authority of 99.
Ahrefs.com's Page Authority is more reliable for these circumstances.
Ultimately, a list of "core keywords" should be limited to approximately 10 in total. Any more than that and you're no longer talking about "core" keywords. So make sure your list also focusses on a fairly broad range of top-level targets. A list of core keywords shouldn't list more than 2 or 3 variations on one type of keyword before moving on to a different top-level topic.
There you have it. Now that you have a list of core keywords, you can make sure your site is organized and built around those keywords.