“Article writing” is basically any short form content. What the client wants is what the writer should focus on. However, your personal style is up to you. For example, I am a pretty conversational writer, and my writing, my blog, and my books all reflect that. So, when a client hires me, they are getting a style that already exists.
Then I write in my style on the topic they want. Articles can be 300+ words, even long form articles of 2000+ words.
This is why there is not a lot of “how to write an article,” because it will be different for different needs. Comparison articles, interview style, product reviews, explanations, stories, etc.
Here are a couple things on relationship building and finding clients: