OneShot: World Machine Edition

In the Glen, after getting the wool from the shepherd, when you offer to trade it with Magpie, say no the first time. He'll then offer the Ram shirt, accept that offer. Once you get to the lower Refuge, once you get in the alley where Niko sees the flash of Rue (the fox), go all the way to the right, equip the shirt, and interact with the wall with the ram symbol. It'll open and when you go in, the achievement will pop.