Enroll in a ballroom teachers training school. There are many schools that are geared just toward teaching ballroom dancing. They have teachers training programs and offer certification for completed courses, which makes you more credible when looking for a teaching position. Some teacher programs also allow you to become a ballroom dance teacher without any prior dance training. These programs will teach you how to execute the steps, lead and follow, and how to teach private and group lessons among other things. There are quite a few websites for this.
Take private lessons. One-on-one instruction is very helpful in learning anything. Although private lessons usually cost more, it may be worth it to you to if you are looking to learn things at a faster pace and with more focused attention. Most dance schools will offer private lessons with some or all of their teachers.
Attend regular dance classes. Any dance training you acquire can help you become a teacher. Note how your dance teacher explains and executes the steps. Many dancers who have professional careers or attend class on a regular basis go on to become teachers. It's an easy transition for some to go from student to teacher, depending on how well they learn and can turn their learning into teaching.
Purchase instructional videos. Not only will you learn the steps, you will see how the instructor breaks down the steps in order to teach them. The best way to learn is in class or a teacher's program, but if neither of those are available in your area, you can start to learn through ballroom dancing instructional videos. Many of these are available in dance stores or online.