Well, first you must know the friend. Know what she likes, how she thinks, what she fears..and determine if you have those qualities. Most girls do not like people who are so shy as to be incapable of voicing their desires. Know her history as well, and watch to see if she wants you. Then you have a choice, you either just go up to her and ask, or you slowly ease your way closer and closer to her.. it is up to you, whichever suits you better.
Just like sanguine said.. you need to make sure she likes you first you don't want to just up and try to get with her you will have your feelings hurt if you find out she is with someone.. She may not want a boyfriend either. & just be nice to her show respect, compliment her tell her she looks nice today. Say hey to her when you see her first