Personally I think the simpler the better. He does not need to be overloaded with information that may have taken you years to process. It should be your words, not ours. I don't think they need the technicalities. He does not need to know about us, all that matters is you.
I would start with saying why you wanted to tell them, and what you hope to get by telling them. It might be you want to get treated differently, or you want support, or you want a better relationship with your dad, that it does not change the fact that they are your dad and that it is hard for you to tell them and why you find it hard etc. Ultimately it is because you care about them.
Some points to make clear: It is not because of anything he did, it has nothing to do with sex. Apart from that all they really need to know is that you feel like you are a boy inside, that you have felt this way for a long time, you struggle with your body, growing up was difficult, there is no 'fix'. That you have a need to feel real.
It might be good to talk about things from your childhood that they can relate to - like the way you love football, or how you hated wearing dresses, so they can see it was something that has always been there and part of who you are and gives them a point of connection.
Hope it all goes well!
Nicki
