I get many unsolicited resumes from copywriters each month. I get asked all the time about how to become a professional writer. I have only ever had two writing rules.
One I made up but share with many other pros: read whatever you’ve just written out loud. The other I got from Stephen King’s “On Writing” - he says that the second draft should always have 10% fewer words, by actual count, than the first draft. Lately, I have modified this to be a 30% reduction, and I am never disappointed with the results.
(Also: if you’re applying for a copywriter job, proofread your cover letter. And resume. Please.)