To unlock the From Niwa with Love achievement in The Medium, you'll need to track down a single postcard from Niwa's former groundskeeper, Frank. There are a few of these to find throughout the game, so we'll just cover the first one available for this one.
After the cutscene where Marianne rides off to Niwa in search of the mysterious man who called her during the game's opening scene, you'll find yourself at an abandoned checkpoint. The gate is locked up tight and to get in, Marianne needs to go round the back and scale the wall with the help of a dumpster. Before that, though, we're going to want to pop into the building itself quickly, as that's where you'll find the first postcard.
There's not a huge amount you can do in this mess of a room, beside looking out the window to see a strange figure shambling off down the road in the distance. Still, there's an open suitcase towards the back of the room, so let's check that.
Bingo! Amid the assorted possessions is the first of several postcards in the game, so just interact with it to add it to your collection. Well, start
your collection, really. If you're into keeping up with The Medium's lore, be sure to give this (and everything else you find) a read by pressing Y — you can also spin it around with the right stick, but the handwriting on the card itself can be kinda hard to make out.
Keep an eye out for more of these little keepsakes on your travels, as you'll need to find all of them for the All That's Left Unsaid achievement and some are a fair way off the beaten path. I'm still missing one or two, but I'll throw up a guide for that one too if I manage to track down the one(s) I'm missing any time soon...