There are two ways to answer this. There is a way to do it if you wish to be on the Alliance, and a way to do it if you want a faster, easier, less expensive way. In general, if you want to teach in yoga studios, and command the respect of the yoga world you will want to go through a training that follows the standards set by the yoga community, through the alliance. If what you want is to be able to integrate a bit of yoga into your training and teaching style there are programs that do not meet those standards, but will give you a bit of basics.
The Alliance does not offer certifications or training: It simply is a neutral body that maintains a list of yoga training programs and yoga teachers that have a certain level of training. So for example an RYT is a registered yoga instructor. An RYT 200 is a registered yoga teacher who has completed a 200 hour program. ERYT 200 or 500 stands for a yoga teacher who has documented experience teaching: either at least 2 years and 1,000 hours, or at least 4 years and 2,000 hours of teaching. I can tell you that if you want to be able to work in most yoga studios it is very helpful to have at least a RYT-200. To get this you need to identify a school or training program which matches your interests and needs. Some programs are residential. I did some training while staying at an Ashram in Santa Barbara, called White Lotus. Some programs are run out of a studio, and may be able to be done on weekends, or evenings.
All of this is relatively new. Not long training; most people used to do yoga for years and years before starting to teach, and studied, and read, and immersed themselves in it. Some who could would go to India to study. This still happens, but more people train in yoga schools without going abroad. And since research began to document some of the benefits of the practice yoga schools have sprung up all over the place to offer training.
What kind of yoga do you want to teach? There are huge differences. Some styles are very big on alignment, some are more athletic, some are done in heated studios, some involve chanting, some inherently include much more than just the physical practice of asana.
What kind of programs are available near you? Do you have funds or time to travel to do a residential training?
If you want a program that does yoga variations, they too abound. There is a market for such training, as people see you can make money teaching, and it is really appealing to get it done quickly and inexpensively. It can be a useful way to begin training, but it will not have the depth of an actual training through one of the major yoga schools.
If you can give me an idea of what sort of yoga you think you would like to study I would be happy to give you some leads to the major styles. Also, if you can let me know yoru geographic area it will help, as I would probably not recommend something in Virginia if you are in Oregon, for example. You can contact me through this site. I also have blogged on how to find a yoga teacher, and you might find that helpful as you think about what program would best suit you.