Send them a link to something that's peripherally related to the subject you wish them to become enamored with. Like a funny image macro or meme, something they can find funny and understand on some level while leaving them ever so slightly confused about its connection to ponies. Maybe even something innocently silly they will enjoy.
And then you wait. It might take a while but the idea is spreading, taking over their psyche. They will begin to recognize ponies here and there; the images and videos will keep replaying in their heads. And at some point they will wonder: "What is it about those ponies?"
And then they will ask you, you who first brought it up. This is critical, don't oversell it; they've let their guard down, they've accepted that maybe, just maybe, there may be something about those ponies. Nothing that's, you know, girly and stuff but something funny, something interesting. They're curious, like a little squirrel slowly approaching a human who's offering food. Don't make any sudden movements. Just mention, innocently and nonchalantly, that there's this show called My Little Pony, which is not like the old MLP shows that, apparently, some people on the internet love.
But you wouldn't know anything about that of course. You've just watched a couple of episodes on YouTube and thought it was, you know, kind of funny. And then you send them a link to one of the better episodes (refer to this thread if you're unsure about which one) and then you've got 'em.
Oh they will hesitate at first, they won't admit that a show about ponies of all things could be awesome. They might even retreat, completely drop the subject. But it's too late. The idea has been planted.
During idle moments they will think "Hey, that Rainbow Dash was kinda cool!" and then they'll realize that they actually remember one of the characters' names. They will curse themselves, they will hate themselves for giving in to something this childish but they know that Rainbow Dash IS awesome. And they will want more. They will seek it out themselves; they won't tell anybody, of course but they will watch every episode and they will fall in love with Twilight Sparkle and Applejack and Pinkie Pie and Rainbow Dash and Fluttershy and Applebloom and Spike and Rarity.
All right, maybe not Rarity but still. And at some point, they will come to you and say "hey, that show is actually kind of funny!". That's when you begin to smile, slowly and subtly; move a little closer to their ear and whisper:
Welcome to the herd, brony