When someone asks how to come out to their friends, “Don’t do it” isn’t particularly helpful advice. I’d recommend AGAINST the “slow burn” approach that some seem to be advocating. Your friends WILL notice and they will likely be a little freaked out by it, especially if you don’t say anything.
The first thing to do is figure out WHICH friends you want to tell. If you just want to live full time en femme, then you’re pretty much going to have to tell everyone. If you can narrow it down to the friends you think you’ll actually dress with/in front of, that will lighten the burden. Once you have your list, just sit each one down one by one and be honest. Tell them that it’s a part of you that’s been with you for a long time and you hope they can accept that part of you in their lives. If they can’t, find better friends.