How do I write a summary in an article?

I am new to writing articles. I have been asked to write two 400 word articles, each with a 20 word summary included. How do I do this? I have searched and can not find an answer. The articles are not for school work. Also, the articles are to be SEO rich, which I understand, but do I include the keywords within the summary? I'm so confused, Help someone! Thanks in advance.