How do I do this? I looked everywhere and spent a couple hours on it. Since it's my first playthrough I just took her up to her room and killed her, but for my next playthrough I'm going to attempt clean hands and figure out how to finish everything without killing anyone.

There is a guy with a creepy scarecrow style mask somewhere. He will give you the non-lethal option.

http://i2.ytimg.com/vi/-Xy-Efg57fI/hqdefault.jpg

^That guy

Where is he located? I looked around the whole party and didn't see him once.

I think he moves around. he actually came to me when I first did it.

Only issue is getting her into the basemant. I have no idea how anyone would do it the non lethal way, if they didnt have possesion rank 2.

I didnt the first time either. If you talk to her and convince her to escourt you to her bedroom, then you knock her out there and walking down the stairs unavoided is quite easy.

I had the option to say "There's someone coming to kill you, go hide in the basemant" essentially. I knocked her out in the basemant and gave her to Brisby. I found out her name prior, if that helps.

Lady Boyle is randomised each time. I had one who wanted to go to the music room, and there were still guests everywhere.
So I just possesed her, and knocked her out in the celler, so much easier. However spring razors are fun, so I wont ever worry about that again.

Mine was the randy one. I found a note in her bedroom where she'd written that she planned to bed the first man who asked her, so I went and asked her. When she led me to the back stairs that apparently triggered the guard on duty there to change from a static position to a roving patrol, so I was able to KO her in the bedroom and carry her down the stairs without that guard in the way.

I KOed her in the bedroom and carried her down the stairs...I met a guard on his way up and he drew his sword, but let me still pass unhindered...not sure if that was a bug or not, though...