Know your stuff. Top magazines don't hire just anyone. If you're a decorated writer or editor, you're only halfway there. Know how to write fashion features just as well as you can write a hard news piece. Be skilled in proofreading, interviewing, and researching.
Build up your resume and portfolio. Compile all of your clips into a binder. Scan them to have on-line copies of your stories, as well. Tidy up your resume and make it relevant to the position you want.
Work in the field. Intern at a publishing house, magazine, newspaper or website. Take a full-time job if possible. Try to get involved with fashion and styling if you're fortunate enough to work in publishing.
Stay knowledgeable in fashion. Dress fabulously. Know the difference between Marc Jabos and Michael Kors. A fashion editor must be schooled in all things fashion.
Attend industry parties. Network. Show your face at fashion week. Introduce yourself to as many style icons and the people who work for them as you can. The key is, get out there and become a recognized face and name.
Contribute freelance fashion columns and articles to a major magazine. Chances are, it won't get published easily. But, if you're serious about entering the field and you're a talented writer, you might just get published.