Patience is the surest, but slowest way
and it all depends on the individual. Some people automatically and adamantly refuse to even consider it, so the first mentions might need to be handled extremely delicately. Most reasonable people can at least consider it, or entertain the thought without actually attempting to practice a form of non-monogamy.
If you meet resistance, the surest way is to not actively pursue other intimate relationships, but pursue close friendships with others, friendships that are not forged with the intent of the relationship eventually becoming sexual. Friendships with people whom are "your kind of people" meaning the type of person you would desire to form strong bonds with. The type of true friends you can confide in and the closeness being based on the fact they are trustworthy.
Those seem to be the friends whom you might "stumble backwards" into polyamory with.
However they are also the kind of friends most people never are blessed by becoming having those types of relationships, the invaluable true friend. They are not a dime a dozen.
If your spouse is honest with himself, he will be able to recognize that he can be susceptible to at least brief moments of attraction or lust towards others, even though it is not a good idea to run with every instance of those emotions as to do so would be careless unless you are single. To run with every lustful emotion is careless, even for polys. Acting on casual lust, is more a swinger type mentality.
Working on fully sharing your thoughts with each other will elevate the closeness or intimacy in your relationship with your spouse. Just being able to discuss casual attractions and lust that you would likely not ever act on is level of intimacy that most marriages never reach.
If you were to foster a deeper connection with your spouse by working towards more fully sharing your life -- regardless of your intentions to practice polyamory -- is in my opinion the surest way to transition into non-monogamy. Spouses with that heightened level of trust, that strength and closeness, that extremely intimate and more complete sharing of yourself with your loved ones is what can make non-monogamy come easy
So sometimes the best way to start, is not bring up sexual relationships with others, but first get him to open up and be comfortable discussing detailed sexual interactions with you.
You would be surprised how many people attempt to be non-monogamous yet the sex acts they envision in their fantasies, they are too embarrassed to discuss those very acts in detail in the context of it happening with their spouse.
They cannot even bring themselves to descriptively talk about it, yet act surprised when their relationships implode or explode, which sounds like denial to me.
that's already way too much information, so a lot of it may be irrelevant, but if something helps, it helps.
hopefully something will help you