[Michele in Kentucky]

My Boyfriend has asked for a threesome for his birthday and wants me to initiate on my own as part of the turn on for him. I have never been in a threesome or had a sexual experience with another woman, but very much want to do this. I don't have a clue how to approach another woman or how to know if she is interested in other women or a threesome. I don't want to go to a bar and pick up someone who does this every night with someone new. How do I approach a woman and how do I know who to approach?