Basics:
The most important thing is that you and your girl both know what you want. Have a good long talk so you both know how you feel about this. Is she worried about anything? How does she want this to happen? I there anything she doesn't want you to do with the other girl (like kissing), or anything she really WANTS to do in a threesome as it would really turn her on? etc etc...
Then I suggest your girlfriend first of all asks the other girl what she thinks about threesomes in general.. your girlfriend should concentrate on dropping hints about a threesome to the other girl... "I have this fantasy... wouldn't it be really hot... etc etc". Make it HER fantasy as well. That way, when it starts happening she will be totally wrapped up in her own experience ("I can't believe they're both touching me.. mm, it feels so good" etc). This is MUCH more important than her having a high attraction to you...
A few pointers:
In my experience (not that I'm an expert, but I HAVE had 3 threesomes) it's more important to put the idea of a threesome in the other girls head than it is to build individual attraction... And when it's showtime comfort is very important, attraction is somewhat secondary (compared to a "normal" one-on-one seduction).
Seduction:
After you're sure you've cleared the air and put the fantasy firmly in the other girls head, just invite her over for dinner and a movie or something... drink some wine or something (just a glass or two though) to loosen up a bit... then proceed with seduction as usual (flirting, kino, light kisses etc). Just remember that this is about the other girl... YOU two want the threesome, so YOU have to combine forces and seduce her. If you manage to get her horny enough I'm sure she'll agree to it.. and by then it will also be pretty obvious that your girlfriend doesn't mind her touching you/you touching her

Getting touched/kissed from two persons at once is an incredible turnon in and by itself.. If you need help getting started, try using the "dual induction massage" routine, which you can find here at the forum..
Good luck! I hope you get your threesome and that it's a good one! A really good threesome is THE SHIT!

Unfortunately, a bad one can be a relationship killer.. so be careful and TALK about your feelings before and after... and (IMPORTANT!) be sure that your girlfriend knows that it's okey to say stop at any time if she should feel uncomfortable.