I think you need to be up front and honest with the other woman about your likes and dislikes. Don't assume that she knows what your wife likes or what you like. Also, make sure you have that "safe word" that if any of the three of you feel uncomfortable, the word is said and everything stops, no questions asked.
As a woman, I have a hard time believing it would be a turn on for her to watch you have sex with another woman. Are you sure about that? Even if she says it's okay, be prepared that there may be feelings of jealousy arising out of this encounter.
Also, I would make sure the woman is tested for diseases and stuff first. Agree on birth control beforehand too.
I have never heard ONE person who's had a threesome who said it was an awesome experience and they can't wait to do it again. Most people say it's awkward, someone always feels left out, and it's not nearly as sexy as porn makes it out to be. Hence, the reason why I usually advise against it.
If you're going to do it anyway, just make sure the lady is clean and into safe sex as well. I would try the internet for what you're looking for. I'm sure there's plenty of people out there looking to be the "other person".