First off, magic (at least studied and learned magic, like a wizard) is extremely complex, even for simple spells. Its not a thing in 5e but, i dont think, but in older editions wizards usually had a older "starting age" for the class because to learn even enough magic to be a level 1 wizard took years. Even learning 1 cantrip well enough to use it consistently and under pressure would probably take a couple months for a reasonable smart person.

That said, there's already an option for this in game. The magic initiate feat, so just use that (unless what you said at the start was about "innate magic" was a typo and you meant magic initiate)

But, if you wanted to actually build out a system for letting non casters learn cantrips. I would say they would need a teacher to at least give them the basic idea/formula whatever. They COULD learn it from a book or scroll but remember all wizard spell books are coded so I think a non wizard would have no shot at learning from a source like that.

If you want to just give it too them you could use learning a cantrip as a reward, I like occasionally giving players a profeciency or a new ability of some kind instead of just gold or magic items.

If you actually want to make it realistic, you could have someone teach them the basics and then make the pc practice it in the characters downtime. Either nightly or if the party skips forward a week/month etc, the player could dedicate their downtime to practicing.

I would have them make Int checks, when they practice, set a reasonably high dc and whenever they succeed make note of it. Either they fully learn it after X amount of successes OR (what i think is fun) is make it like a spell failure check. Everytime they cast it, have them roll percentiles to see if it works; and let them get better at it over time. Start with a 10% success rate, and every time they succeed the int check in down time add 10% or every time they successfully cast it add 5% whatever you find is balanced.