This is an important step in the SEO exercise. What keywords do you need to emphasize and include? How do you choose the most relevant keywords that will be used by your target audience? Pose yourself the question: What would my target visitor be looking for and for which queries I would like to lure him/her to my site? Imagine those queries, as many of them as you can think. Apply this to all categories of visitors that you are targeting. Then list those queries and formulate your keywords from those.
Let your creativity develop grammar and synonym variants of these keywords. The same would apply to descriptions; however while keywords have to match closely with search strings, descriptions should be drafted to allure the visitor after he has seen your listing. It should tell the visitor that your site is indeed offering him best information or outcome for what he is looking.
Select phrases (at least two words), rather than single words, as there would be too many contenders for single word searches. You are much better off focusing on specific search keywords and using longer phrases.

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