I just approached them, I did so without knowing what to say. I can assure you that when you've come far enough to walk up to her it will be too embarrassing to walk away, your mind will use all it's strength to avoid it and will do so by giving you something to say. This usually ends in being an indirect approach.
For the direct approaches you should prepare a little. You gotta be prepared for any reaction and feel confident that it won't negatively affect you. So even if she straight down reject you, you must know you can laugh it off and not let it hurt your ego. This is where outcome independence matters most.
What approach you use isn't really that important.