Fallout 4
84 Achievements
1,600
108-139h
Ranger Corps
Discover 100 Locations
40
20.58%
How to unlock the Ranger Corps achievement in Fallout 4 - Definitive Guide
http://fallout4map.de/w/index.html#3/-66.48/-78.84
I am putting this here as a reference. It's a neat interactive map that Reddit has offered up, and it shows the locations of all key collectibles in the game. What I would suggest is going for the bobbleheads, magazines, giant creatures etc etc, and then heading into the inner city to mop up the rest of your locations. It really isn't much of a grind, and can actually just be gotten over time if you finish the game. If you really want to grind it, though, this is a good way to go.
But just like you said, this should unlock by just playing through the game.
With 330+ locations in the game this achievement will not be something you worry about unlocking. Going into the actual city of Boston and exploring all of the unknown locations can net you upwards of 40 locations within the city limits.
For this trophy you simply need to discover 100 locations. You will notice as you are walking around the wasteland that a message saying "You have discovered X!" sometimes appears, this means you have discovered the location you have just entered and it'll be added to your Pip-Boy map. This also allows you to travel to the area via Fast Travel or Vertibird. You need to find 100 different locations for this trophy but there are well over 200 locations on the map so you should just get this one over time by playing the game.