Talk to Anora in Eamon's estate in Denerim after you've rescued her. 1. Just pick a dialogue line then 2. will be available, quite easy. 2. "I think I would need more than just your support." 3. "What if you had a strong king beside you." 4. "(Persuade) Marry me, and you will have my support."
Also, you need to be a Human Noble male and not kill Loghain.