When you meet her in the beginning, talk to her and ask what someone like her would be doing in Lothering, she will then ask you what you mean by that, then you ask her what someone so beautiful would be doing in the Chantry, then she will ask you if you really didn't think there were any beautiful women in general in the Chantry, you answer that they couldn't have been more beautiful than her. Thus your romance is initiated. To continue you simply have to shower her with gifts(basically everythin Andraste related), be a good guy, flatter her whenever you can and so on.
There are hindrances. Don't go too far without the Cute Nug or the quest about Marjolaine, or a 'real romance' won't occur.