The current process that most Red Cross chapters require is thus. First, you have to take the "Fundamentals of Instructor Training" (FIT) course which sounds like what the Goddess mentioned. Its a short class that basically introduces you to how the ARC works and the basic procedural requirements for all their classes (how to complete the training record, how to conduct tests and evaluations, etc.). After you take that, you can take the instructor class for whatever course(s) you want to teach.
The basic Community First Aid/CPR instructor class is, I believe, 20 hours long. You only have to have a first aid/CPR cert to take that (along with FIT). AED may or may not be included in that depending on the Chapter, or may be a stand-alone extra course.
The Emergency Response instructor class is the same length as the basic class. ER is the ARC's First Responder course. To be an instructor for that, you have to have at least the ARC ER certificate, but since EMT is more advanced, they'll accept that too. Once you become an ER instructor, you're authorized to teach ALL levels of first aid/CPR/AED from the basic adult CPR class up through the full-blown ER class.
Only trouble is that while most ARC chapters offer the basic instructor class, not all of them offer the ER instructor class. When I did mine, there were only 2 chapters in the area (out of 10-12 or more total) that offered it, and those only offer it once or twice a year.
The other option is that you can do the basic instructor class and then later take a "bridge" class to upgrade to the full ER instructor, but my opinion is that its easier to just start at the highest level.
Once you're an authorized instructor, you have to teach at least one class every two years to maintain your authorization. That means that if you're an ER instructor, you have to teach (or co-teach) a full ER class, regardless of how many other classes you teach.