One typical cuckold wannabe strategy is "slowly getting her ready" - by little encouragements here and there, casual mentioning of how it turns you on once in a while etc.
I believe this strategy is useless, especially if your wife is a real conservative type.
If you are willing to take some risk, here is your best option: Arrange with a guy (could be a bull you meet online, could be a guy you just met on vacation) before wife knows anything about it. Make sure the guy is someone who really wants to fuck your wife, someone who is for sure better at fucking than you, and someone who is willing to work with you in turning her into a true cock lover and you into her lil cuckold.
If you can, go away on vacation and make sure he is there also. While away from home, your wife is likely to lose some inhibitions. Three of you should spend a few days together, with you encouraging at every step of the way that the two of them are close. For example if you go somewhere that rents jet-skis (only two people on it), make sure the two of them go and do it while you sit ant watch. Try to make this happen as much as possible for the whole time.
(if playing pool, the two of them are on one team, they play another couple, you get the takes)
anything else you can think of that will give your wife the feeling that he is acting as her partner. this may make her see him in a different way. some conservative women do not even see men other than their husband in a sexual way. but the more she is with him, in a way him acting as her man, she might start getting wet, even just a little, when his body rubs agains hers and when his arm bumps into her chest.
After this initial prep, I say you gotta bite the bullet, get the three of you in a place where they can fuck (hotel room ideal), and bring up the subject. if you are luck once he starts flirting with her, she wont put up a fight and maybe you can sneak out for a bit while they ease the tension, and you can stroll back in when she is spread eagle on the couch.
good luck.