By accident, mostly.
You might think that’s a joke, but I’m actually dead serious. Eevee are fascinatingly adaptive, and as such, they respond to stimuli we never would have imagined would trigger evolution. For example, who knew that feeding your eevee poképuffs from a Lumiose bakery and spoiling it to ridiculous degrees via daily grooming would cause it to evolve into sylveon? Or training it in a specific forest or icy cave would help it evolve into leafeon or glaceon? (I mean, granted, yes, it’s adaptive, so one would assume training it in specific locations would trigger those evolutions, but the point is that not every forest triggers an evolution into leafeon, for example.)
The funny truth is that the harder researchers work to discover new evolutions of eevee, the less likely we’ll actually find them. It’s far more likely that one of us will simply accidentally dump a metal coat on an eevee and wind up with a steel-type eeveelution.
Incidentally, no, that doesn’t work. I might have tried. The eevee in question was not amused.
(But then again, maybe it’s dump a metal coat onto your eevee and then do something else with it. Hmm. That … that might need a follow-up there.)
The Pokédex According to Bill 