There are a total of 30 Outhouses you have to discover. When you walk up to an Outhouse, it will say activated, but you have to press on it to fully 'discover' it. Outhouses are large brown buildings that are mostly found on the side of roads and usually right before a difficult part of an area. They're not hard to spot and you can easily see them in the area maps. Don't be mistaken about the Outhouses inside any demon caves. Since they're Out-Of-Order, they don't count for this achievement. All Outhouses that you need for the achievement are out in the open world.
Map showing all locations of the Outhouses: Link