For this achievement, you have to use the "steal map" covert action. To gain this action, you must have diplomatic technology of at least 6, and need to build up a spy network of size 50 with the country you wish to steal maps from. Keep in mind you need a unit in a region adjacent to the region map you want to steal. For example, if I want to steal the Brazil map, I need to have units in Columbia, Upper Peru, La Plata, or in the South Atlantic.
Getting this achievement is easy so long as you are a colonizer or in the Native American region at the start of the game. Just build up your spy network, get your units into an adjacent region to what you want to know about, and then steal the map.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.