Three some advice and tips?!?

My boyfriend and I have decided to have a 3some. We are both going to find a woman and pick between the 2. I’m just not sure how to go about finding a woman. I’m the one that wanted the 3some cause it’s something I’ve always wanted to try and he agreed to do it.

Any tips on how i should approach another woman about being our third. I know i will have to flirt with her I’m just not sure what else i should say before asking her to join us.