Here's how I do it:
Invite them out for coffee or smoothies. Let them tell you about their day first, then say you have something to tell them. Once you have the floor, just explain yourself as best as you can. It's scary, and you have every right to feel nervous, but it'll be great when you can finally get it out there. It really isn't as bad as you might imagine. At least, nobody has reacted negatively in my experience. (Except for my mom, but I'll talk about that in a minute.)
Be prepared to have the same conversation multiple times with the same person. Maybe it depends on how much they already know about ->-bleeped-<-, but none of my friends did, so they had to learn a lot to be able to comprehend what I was getting at. It helps if you're able to get them to relate to you.
Examples:
Before I ever came out to my brother as transgender (in fact, before I even knew what "transgender" meant), I found an old letter he wrote to a pen pal in which he described me as his older brother. That made me feel awesome.
A friend I came out to was having a hard time understanding things. I probably only confused him even more every time I tried to explain myself. A few weeks later, I remembered this story and told it to him--he totally got it. He was the first person who made the leap from "Okay, so you don't feel like your natal sex?" to "Oh...so you probably hate being referred to with female pronouns, then." People probably aren't going to reach that conclusion by themselves unless you guide them there, so don't be afraid to do that.
There were two ways which helped me to get through to my mom. First, I asked how she'd feel if she went out in public and everyone started referring to her with male pronouns. She said it would make her mad, and when I asked why, she said, "Because I'm not a boy." I said, "Exactly." Her immediate reaction was, "Well, neither are--" and then fell silent as she made some sort of connection.
Also, I asked her to imagine her telling my brother, verbatim, what she kept telling me. She was saying things like, "You should remain a girl. Do whatever you need in private to feel like a boy, but don't try to change who you were born as." She thought it would be silly to say those exact things to my brother, "because he isn't a girl." "Yeah, Mom, that's what I mean." And again, another moment of realization.
If your friend is responding negatively, maybe try those techniques, replacing "brother" with a mutual male friend or something.
All in all, just keep a positive attitude. Things should go smoothly. It's possible some of your friends might already suspect something's amiss and are just waiting for you to disclose (I got this reaction too). They're your friends, after all, so they should be willing to talk it out with you!
Good luck! Keep us updated.
