You'll have to do her personal quests. Travel with her in Orzammar, talk to her - while in Orzammar - about nugs. Get a cute nug from Idle Dwarf in Dust Town.
Give Leliana the nug, and then travel with her a bit. If you're already 'together' with her you have to break up with her.
Travel until you get her personal cutscene fight, and talk about Orlais/Marjolaine. Actually look up the Marjolaine questline for that.
Go see Marjolaine, finish that and then talk to her again. Pick the 'I wish someday I could be that special to you' option.
It can be locked off if you kiss her before the nug-quest/Marjolaine questline, so it's a bit tricky.