When you succeed you’ll have to get permission from her father, or other notable clan members. When you return you’ll need to do another charm mini-game (make sure to save). If you succeed you need to leave and wait at least two days before returning.
If you fail just reload the previous save and try again. To win just pick the options with the highest success rates. Before talking to her make sure to save (in case you fail the charm mini-game) When you talk to her choose the option to profess yourself her Admirer, this will lead to a charm mini-game. First, pick an eligible lady and use the encyclopedia (‘N’ Key on world map) to find her. Wives with good stats can lead clan parties, govern castles and settlements, lead caravans, be companions in your party, and provide children (this feature seems to be incomplete and useless). Once you’ve built a small warband, you’ll want to take advantage of marriage.
