Each state and local area has slightly different rules and regs. Let me tell you how I did it in Georgia.
I contacted a Rape Crisis Center in my town. They screened me and put me in touch with a state wide program out of Atlanta, Georgia Network to End Sexual Assault (GNESA). I attended a class that was one week long, and very intense. Covered topics such as understanding the victim, communication, details of the exam, etc...
Following completion of the class, I had a 40 hour preceptorship to complete before I was certified by GNESA as a SANE. The 40 hours could be scheduled at my liberty, and included: hours with an NP pr MD learning to do pelvic examinations, hours with a pediatrician learning pediatric exam techniques (I clepped this, as I am a certified pediatric nurse), doing an 8 hour ride-along with local police (to help us to understand law enforcement's role, as we would be working together), time with the local DA who prosecutes the cases, time with victim assistance services at the courthouse, time spent observing court room trials (as we will be called as the expert to testify if our case goes to court), time spent with rape crisis center to learn what services they offer to the victim, and time spent with the state bureau of investigation to see how DNA is processed. Also, I had to do 3 supervised (precepted) exams, so I had to take call with a SANE preceptor until I fulfilled that requirement.
The basic requirement is to have two or more years of experience before being admitted to a SANE training program. Many local DA's offices fund the training, and some rape crisis centers do as well.
There are degrees offered in forensic nursing at the master's level, but you don't have to have one of those to work as a SANE.
Look at the website for the International Association of Forensic Nurses (IAFN). www.iafn.org
Hope this informaiton helps!