For the love of god, don't be these people:
Further, there are some good thoughts here, from the perspective of the potential new person, that might also be helpful for you: http://www.morethantwo.com/coupledating.html
Beyond that, be polite, completely honest and open, and forward without being pressure-y. My gf's husband did the same thing you're doing and two years later she and I are still together, so it *can* work.
Also, know your audience... his approach might be too strong for some people, but he knew I was a sexually adventurous person and I'd expressed interest in them many years ago in college, so he was fairly sure I wouldn't take it amiss. He basically just said "Hey, Gia and I are finally over the debacle that was our last gf and we're ready to try opening or marriage again. You're at the top of our list for people we think are really hot and awesome and sane. Would you be interested in a threesome? Maybe Thursday? No worries if not, we'll find someone else to ask." No assumption that I'd date one or both of them, no pretenses that this was really all about me and that they weren't looking generally, just casual hot times to start... but it ended up turning into something pretty deep for she and I.