Being that she is your supervisor, it is unlikely that she would make a move on you even if she likes you, out of fear of jeopardizing or taking advantage of her senior position.
Thereby, if you hope for anything to come out of this, you would have to be the one to make the move.
However, if your work has strict rules about asking a coworker out, you should perhaps ask yourself about whether you are willing to sacrifice your job in order to take a chance on being with her.
If you feel that you are willing to do that, you should tell her.
If you choose to tell her, just keep it brief.
Tell her that you think she is pretty and would like to get to know her better over coffee, lunch, a drink, etc.
Avoid saying too much. You could get lost with your words if you talk too much, to the point where it just feels awkward.
Keep it brief and just let her give you her answer.