There is no “one size fits all”.
start by asking your partner if they have any bedroom fantasies or a fantasy bucket list they are open to share. This starts the conversation. If the idea of this conversation is confronting to ur partner, the potential for a 3rd is pretty remote. You have to overcome the insecurity associated with the idea that while you are devoted, you still have fantasies that if realised could make the existing relationship stronger, rather than break it into 1000 pieces. The idea that you want a 3rd, could initially suggest that your partner does not satisfy u, which could be confronting to their self esteem and be interpreted that there is a risk you will cheat to fulfil that desire/need!
How you handle things from there depends on the conversation. To be successful, you need to ensure ur partner has some ownership of the idea/outcome/fantasy. If they just go with it to satisfy you, but have no buy in themselves, it will go belly up.
if the fantasy discussion has legs, you could suggest an exploratory visit to a swinger’s club just to “see how the other half lives” but with an agreement not to indulge before you download from this research visit. Then agree ground rule including agreed backup signals if anyone changes their mind mid stream.
