If you are telling your SO's parents, their immediate concern will be her welfare. It will be important to reassure them that she is good, sticking with you, and her feelings are being respected. Parents don't like their child hurt. It may even be something you want to say up front.
I have always tried to lead up to any announcement like, "I am trans" with a bit of setup in the form of my history. This included, for me, the pain involved with dealing with this issue. Once they hear the announcement they will probably hear nothing else after. I found it helpful to tell them the choice being faced. "I can continue on my current path and be absolutely miserable, or I can make a very scary change that will give me the chance to be happy." Saying that shows that this is life and death for you, not some whim. I don't know if any of this is useful to you but I have had good feedback from my coming out. I don't know about your history, but I had in my mind (when I told someone) that no one fought against this change happening more than me (I hid it over 50 years). I kinda felt like, "You, person who I am telling, You want to be resistant or negative. You have nothing on the feelings I have had in my past. Get over it."
Good luck, Rambler!
Moni