Keywords are very tricky and you should only offer them if you really know how to use them. (That said, most buyers don’t have a clue.)
Through experimentation and evolution of my own strategy what I do is:
- Open an incognito window in chrome and search for the highest ranking pages in the same niche as a clients website
- I then take the Urls from these sites and use Google Adwords to search for relevant, low competition but often searched for keyphrases, relevant to the topic at hand
- Google will also give you details of similar keyphrases used in searches right from the search page
- You can also use software to analyze what keywords other sites are already using
The main thing is to be able to use any keywords naturally and not depend on them as a ranking factor. Other things like referring to relevant brands, influencers, and recent industry news when discussing a subject, are much more important.
Then you have things like creating content clusters rather than single standalone articles, focusing on how content is structured and presented, and most importantly of all, having an end website which is optimized for the search.
In short, you will need to research, practice, and develop your own strategy. The best way to do this is to build your own webisite. However, you can also use platforms like Medium and Steemit to fine tune your own blogging/article writing SEO prowess.
Lastly, it is important to not try and over-sell keywords. The majority of my clients have really abysmal websites which really need completely redesigning before they will ever see resutls. In this case, it is (in my opinion) best to include keyword research as standard (if you do offer it) rather than as a standalone extra. - This and always turn away anyone who implies that after publishing an piece of work, they expect to suddenly have Google push them to the number one spot and be hit with a sudden sales deluge.