Seems straightforward. When you are six months out from your availability date shoot them an email and start the process.
I know previously, once hired you could defer a class date for unto a year. - id imagine thats still true today. Seniority being everything, if was me, id email them exactly 6 months out. -and then be in the first class i could.. be it 6 months, 6 1/2, 7 etc.