[MFF] setting up first threesome

Me and my girlfriend of 6 years have been having great sex lately. One night during sex she asked me if I would like to have a 3 some which caught me a little off guard, But during the heat of the moment I said sure why not if you want to. Since then it’s been brought up during sex a few times and we fantasize about it. She seems to get turned on by the idea of fucking another girl and watching me fuck another girl and honestly I do too. I know she has had a little girl on girl action before, We have had discussions about boundaries and what we would like to do to each other and are now looking for our possible 3rd. We love each other and I’m comfortable with the idea if she is and so far she seems very open to bringing another girl in the mix. Any advice? We are both nervous but think it could be fun. Also talking about seems to have brought us closer. We have two girls in mind, one of them my girl is attracted to more but is a bit shy and on the fence but they’ve traded pussy pics, the other girl I find equally attractive and say she would definitely do it but has a bit of more dominant personality and is a freak from what I’ve heard. Any help to have this play out smoothly