Sam starts a podcast for hunter lore and publishes it under the paranormal fiction genre. It's mostly for the benefit of updating hunters' access to information, but he figures if it's available publicly it might end up helping a civilian, too.
It gains a cult following, and soon his fandom is made up of hunters, fiction enthusiasts, and regular believers and want-to-believers in the supernatural.
Dean finds out about it when Claire brings it up to justify her lack of stealth about the supernatural on a case. And at first Dean's upset, of course, but not because of Sam's indiscretion.
"I can't believe you're doing that without me. You'd have more followers if you did your show with me. I'm funnier than you, and all those true crime thingies are under comedy, right?"
"Yeah, but Dean, this isn't true crime. Ostensibly, it's just fiction. I don't even list it under comedy."
And Dean just looks at him until Sam sighs and acquiesces.
Dean's lame commentary on his cameo episode is Not Funny, Sam thinks, and is therefore shocked when his follower count jumps with the episode and social media is flooded with fanart of Sam and Dean despite no one knowing what they look like. They range from Just Some Guys to "fancasts" of actors to eldritch-like beings (a la welcome to night vale fanart).
The podcast is titled "Hunter's Lore," but after Dean becomes a regular and the style of the podcasts become a little more conversational, the fandom starts to subtitle it "so get this."
There are fan communities; at least one really fucking detailed fansite "decoding" the lore; another similar website that expands the canon of the podcast, creating a rich, fictional backstory about Sam and Dean being like Giles and Buffy; and yet ANOTHER website tinhatting the fuck out of them--it's full of shippy fanart that incorporates quotes of banter from the show.
People who know about Carver Edlund's Supernatural books think Sam and Dean are using aliases and that the show is entirely meta. There's a split in fandom about whether that's too cringe.
Dean is almost completely unaware of the fandom; especially as it brushes against the Edlund fandom. (Y'know terminally online? He's terminally offline) So when they meet new hunters he's not shy about bragging about his little brother's hit podcast.