Becoming a good line editor takes practice, patience, and time. Developing the skills of an effective line editor requires hard work and dedication, but can be incredibly rewarding if done correctly.
To become a good line editor, first and foremost, you should be familiar with the fundamentals of language and the rules of grammar and punctuation that are the foundation for effective editing. You should also understand the different types of writing styles and genres. This will enable you to understand the particular needs of the writing that you are working with.
The next step is developing a keen eye for detail. This means reading the text multiple times, paying attention to every word, phrase, and sentence, and making sure that everything follows a consistent style, feels natural, and flows well.
Be sure to continually practice and refine your skills. Familiarize yourself with the tools of the trade and use them whenever possible. Understand the style guides out there and learn them so you can quickly edit and proofread any given document.
Finally, be open to feedback. Have an open ear to other editors and to the authors you work with. This will help you become more aware of the nuances of writing and what your particular style is. This will also give you opportunities to learn and grow as an editor.
All in all, becoming a good line editor requires learning the fundamentals, honing in on your attention to detail, continually practicing, and keeping an open mind. With enough effort, you will soon be able to create great pieces of writing with your editing skills.