So just stick to romantic ladies for this. When I did it with a romantic lady, I was given the persuasion check at 14 relation. I had a conventional lady at 40 and couldn't elope, nor was I even given the option to pass a persuasion check. Ladies that are conventional or moralistic either require ridiculously high relations to elope, or won't do it at all from what I can tell. Ask them about each eligible maiden and make a note of which ones he describes as romantic (EDIT : I'm playing the WIN10 version and I was able to get it with a woman that was 'wild and likes to ride' or whatever, this seemed to be easier for me because these women were more likely to show up to feasts than romantic women). Next you want to talk to one of the bard type NPCs that appear in towns, like wandering ashik, troubador, etc. Garrison some troops in it and the enemy will generally leave it alone for easier targets.

Once you have it make sure you don't lose it, obviously. 95% of the time the king will award it to you. Take a castle or town yourself and then choose to ask for it. The simplest way to do this would be to get 150 renown and ask a king for a fief.

The first thing you want to do is get a castle or town. This is kind of like marrying someone, but the main difference is that instead of getting her relative's approval, you say fuck them and get married anyway. It appears this can only be acquired as a male.