I acknowledge where they are coming from, which is usually like "wtf?!?!?!" and I generally say something like this:
"So, I know this is going to sound crazy and I don't really having a way of making you understand how this feels unless you've been there yourself. Basically, for as long as I can remember, I felt like I was a girl. I don't really know what that means, only how it feels. The person you know now, like you said you didn't know I was trans, that's good, because I don't feel trans. I'm living my life very happily and authentically. You want me to be happy, right? Like I'm not hurting anyone, just living how I am, being authentic, because this is my reality. You have any questions?"
Then we talk, they are usually like "Oh, well, I don't really see you as trans, you are just a regular girl to me" or something and I'm all like "good, that's what I am to me too! So let's order!" 🙂