How become more than friends with a girl?

First of all, notice that I'm not asking this question in a general matter, but for a specific situation. My therapist have introduced me to her niece in hopes we would get along and befriend each other, so we went on a walk I talked a lot. I tought she was a very interisting girl, but since she had a boyfriend at the time, I did not make a move on her nor showed any interest. Now, her boyfriend broke up with her a couple weeks ago, and we have met each other a few more times. My question is, since she is now single and we heave consoldidated a friendship, how can I brake this paradigm and show my interest on her? I couldn't simply as her to hang out, since we have done that before as friends, so she will think it's just a casual meeting again. What do you people think? Please keep in mind we have no friends in common and go to different schools, but her therapist told me she (her niece) thinks I'm nice, smart, interesting and good-looking, so I guess things could work. Thanks!
How become more than friends with a girl?
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