Answer:
In short, some would say: You don't, that's just stupid.
Going into more depth... Well, what has been your relationship with this woman? Have you two spoken about her leaving her marriage and getting together? Or is this a spontaneous thing you want to do? Are you trying to be romantic and sweep her off her feet?
Have you recognized how much is involved for her to leave her current situation?
Since it's so complex, it's more than a marriage proposal... It's a serious heart to heart conversation you need to have. She may have lead you to believe that she's into the affair for the long-haul, but when put forth with the official question of commitment, her thoughts may differ.
Basically - you have to talk it out.