Go to an accredited school for vocal training. In order to teach, you have to learn the art. To become a voice coach it is also a good idea to get trained on different teaching methods. Try your hand at student teaching before taking the plunge into teaching full time.
Get experience with using your voice in a choir or other professional setting. When a student is looking for a coach, they will want someone with lots of credentials. Become an experienced singer, and then become a voice coach. Get work with commercials, recording artists or in the theater. Paying jobs will help you get other jobs and more credentials.
Nurture a passion for helping people. When a student comes to vocal training, they need to feel comfortable with their coach. Comfort is developed by a teacher who is willing to work with someone to reach a trust and rapport. A good teacher will have a personal stake in their student's success.
Connect yourself with a school, or start your own coaching group. Once you have been trained to become a voice coach, you will need to have a venue in which to teach. If a professional school or teaching atmosphere is not your thing, then branch out on your own and find a space and students to coach.