My husband told his parents. He called them (thankfully they live many states away) and just explained the situation to them calmly. He then emailed them resources after the phone call (pflag brochure, etc). I then called them to tell my side of the story. The gist of the conversation was that I wish to remain a part of their lives, I know this is a shock, please ask me any questions, that sort of thing.
In the end, they flipped out and cut us both out of their lives (me for being different, and their son for supporting me). I knew from the start that they weren't going to take it well, but it was something that had to be done. My husband has been on my side 100% and hopes that some day his parents will come around and step out of their narrow little world-view, but we have each other and that's what matters.
If your in-laws are anything like mine, they are going to be (understandably) more concerned about their child than you, so be prepared to explain how this change is going to effect your SO and what it means for your relationship. For example, my father-in-law refused to wrap his head around the fact that his son is gay (or at least that our relationship looks gay to outsiders). I think that 'side-effect' bothered him a lot more than the underlying cause of me being trans.
It sounds like you have a good feeling about your in-laws, and that's a really good sign. Just be kind, patient, and straightforward with them. Like others have said, make sure they know they can ask you questions. Also give them time. You don't have to have a huge lengthy discussion about all the ins and outs of trans at once. Introduce the subject, let them mull it over for a few days, and then ask them if they have questions.
Good luck!