I agree with keeping it short and to the point. That was the advice my therapist gave me and what I did with both my parents, I actually came out to my father yesterday and it went good.
I do have to say I had a strategy before telling them face to face: I first came out to my brother, who I knew would be 100% supportive, and then told him to tell my mom as gossip, as if I didn't know he was going to tell her. Then I waited 2 weeks to finally speak to my mom face to face (this amount of time wasn't planned I just couldn't bring myself to do it) but it was the best I could've done, by then she had had enough time to digest the news and she was more open about it. I also told my brother to send her some transgender related info, PDf files etc that I found online that could help her understand better. Then last weekend my mom was like "oh by the way, your dad already knows, I told him. Please talk to him". So I finally did last night and also had already had time to digest it.
What wording did I use? I went to straight to the point: "Dad I've been going to therapy the past few months cuz I'm transgender, which means I'm going to change my gender. I want to be a woman" (this words might not be technically right but the point is to say it in a way that they'll understand). Short and easy to understand. I also offered them both the chance to go to a therapy session with me, my mom went already.
If you don't have any siblings or anyone to do for you what my brother did for me, you could try an e-mail or text message to give your parents a heads up. Also keep it short, and tell them you'll talk about it once you'll see each other on Tuesday.
Keep in mind that even though this is not new for you, it is super new to them so they will likely have lots of questions and you might even feel like their judging you or not understanding you. That's OK, be patient and explain things calmly. It's also important to make them understand that this has been in your mind for a while, that is not like you just woke up one day and said "hey I wanna have boobs".
Tell us how it goes!