advice on threesomes?

My boyfriend and I have been together for almost 7 years now we've talked about the possibility of having a threesome a couple of times now. I'll admit I'm a bit nervous about the whole thing but he has been nothing but respectful about it. He makes it a point to let me know I am and always will be #1 and that this would just be something we could do together and just have some fun. I am very grateful he doesn't make me feel pressured to do anything I don't want to do. Recently I've been thinking about it a lot more and I'll admit I am more curious about what it would be like to kiss/make out with another female. But I'm just not sure how to get to the point where I'm 100% comfortable. Any advice would be much appreciated 😊