There is no one definitive answer to this question. Depending on your level of experience and expertise, you may want to consider taking a class, working with a copy editor, or self-studying.
If you're starting from scratch, taking a class might be the best way to get a comprehensive introduction to copy editing. Look for classes offered by community colleges, continuing education programs, or online providers. Once you have a basic understanding of copy editing principles, you can start practicing on your own.
If you already have some experience with editing, working with a copy editor can help you hone your skills. Many editors offer free or low-cost consultations, so you can get a sense of their style and approach before committing to anything. Once you find an editor you're comfortable working with, you can discuss rates and project details.
Finally, if you're the self-motivated type, you can learn copy editing by studying relevant books and articles, attending webinars and conferences, or joining an online community of editors. Whichever route you choose, be prepared to put in some time and effort – learning to edit well is an ongoing process.