Ok, I really have no idea the specifics of NFP. I know that it has to do with timing your periods and ovulation to not get pregnant. Obviously, I don't know enough about it, because we had two kids while I was thinking I was not ovulating. The only piece of advice I have, is to really KNOW when you are ovulation, not just when "most" women ovulate in relation to their cycle. I conceived my oldest ONE DAY after my period. Not even 24 hours. That was not a normal ovulation cycle, and up to that point, my period was very regular - very consistent. So, I'm just warning you that you can plan all you want, but if it's not already suggested within the class or the website mentioned by the previous posts, I would invest in an ovulation predictor. Just to make sure you aren't one of those women who ovulate at weird times.
*My pregnancy could have been a fluke, sure. But, my daughter was conceived less than a week after my period, three years later. Again, didn't think I was ovulating less than a week after a period.
Good luck!