This is a super fun part I gotta say. I love making things look old and worn.
How I've been doing this, is just thinning acrylic paints and rubbing the watered down paints all over the walls, using black, brown and a yellowish tone I don't remember what that's called, burnt sienna maybe? Ohcre..ra....something. I've also been making floors for living room and library areas and they look really nice, I've been making the wallpapers of living room look super old and disgusting as well. I don't have photos of that yet but I will post photos about EVERYTHING eventually anyways haha. But yeah I'm getting very excited, it's starting to look like I was hoping it would look like. Just gotta make roof shingles, because I can not be arsed with using the shingles I got with the house, I'm so done with that thin wood that bends and curves from the slightest amount of moisture. And of course to paint the wooden parts of the house outside and adding moss and so on and maybe some plants to the walls, anything to make it all haunted and witchy! :)