I think the important thing is to let it grow naturally (with a few sly moves thrown in, once in a while)
You've been friends forever and hang out and enjoy each others company and all. Don't change any of that. Sometimes, dropping the "L" bomb out of the blue screws it up.
Keep doing what you're doing but start doing special little things for her, like a guy would do for his G/F. Send her a cutesie card for no reason and send her some flowers for her birfday. Buy her a funny little gift once in a while.
Occasionally, and I DO mean occasionally, introduce her to someone as your girlfirend. Just let it slip but don't correct it. Just say it and continue talking.
If she says anything, just say "oh, I didn't realize I said that. It just feels like you're my g/f and I really like that feeling."
If you don't already, hold her hand. When you're walking or watching tv or driving, just reach over and hold her hand. No palm tickling or anything weird, just hold hands.
Pretty soon, I think, you'll know where things are going.
I can't predict what'll happen after you did what you did but I really wish for the best outcome for you.