How to explain to your parents that you're moving out at 18

I know the title sounds simple, but the whole situation isn’t.
The whole question is: How can you start explaining (start a conversation and end) that you are moving out when you’re 18 to live with a friend.
I know that you’re legally an adult when you’re 18, and you can do whatever you please, but I’d rather this go down with the least amount of conflict and drama as possible. Here’s the situation: My friend and I are going to get an apartment when we finish high school and go to college together. I trust her with all my heart and she is not the person to skip out on rent, so that isn’t a problem. But during our senior year in high school, she is going to move in with me and my mom and step dad. My parents are perfectly okay with it and love her just like their own daughter, so they have no problem whatsoever. Since we’re going to be together for that time, we can see what it’s like living with each other and we can easily plan for our apartment and college in the future. My friend is having a ton a problems with her mother ever since she remarried, and they’ve just gotten worse and worse. What is a good way to start up and finish a converstation letting her mom know that she will be moving out when she’s 18? Her mom is not exactly the type to easily let her go like that and she is more than likely to get overly upset. (I’d hate to admit, but she’s an adult drama queen..) I want this to go very calmly and smoothly… Any suggestions would be highly appreciated.