It's FSDO (Flight Safety District Office).
Actually, it's Flight STANDARDS distrct office. I'm not usually pedantic - but you started it.
spils, the FAA website explains exactly what you need to do - it's quite simply if you read it carefully and do exactly what it says. Can't be bothered posting a link - you'll probably find it in the thread above or one of several others. Don't worry about the 3 month (maximum) timescale they quote to process the application - mine was ready for collection in less that 2 weeks. Good luck.