How to start?

We are a couple in our early 40s, married for 17 years, with children. When my sexual life became monotonous, we started to fantasize, and watching a couple making love became our most enjoyable fantasy. However, since we were shy, we did not have any experience. My wife says that if we go abroad or there are people we don't know, we can make love even with your own spouses in the same room, but she still doesn't dare. I would like to chat with a couple who have experience about how we can bring this to life. I say I want it because my wife does not accept it even though she wants it very much.