This guide will assist you in figuring out how to find any missing rooms that you haven't lit up yet.

NOTE: The achievement will not pop until you have 100% and kill the boss.

You will require a semi-decent photo editor, I used Photoshop, not sure what other tools it will work on.

1. First thing you'll need to do is find an image online of a 100% completed map, my screenshot attached here is 100% (except for the Tower Room, which doesn't count for completion).
   
   
   
   
   
2. Next, you'll need a screenshot of your own map. Press escape to bring up the menu/map and press the Screenshot key (default F12).
   *NOTE* Both images, the 100% and your own should be the same resolution to be able to see the differences. My screenshots are 640x480 because that was the default resolution of the game
   
   
   
   
3. Drag both of these items into your photo editor
   
   
   
   
   
4. Copy one image on top of the other. You should see them both on top of each other on the layers area
   
   
   
   
   
   
5. Toggle layer visibility on and off (in Photoshop, this can be done with the eye icon), the differences between both layers should be readily apparant (see gif below).
   
   
   
   [img]
   https://media.giphy.com/media/9ruVLUioKiDdTho6s9/giphy.gif[/img]
   
   
   
   Another method to use is the 'Difference' Blend Mode, this will strip the identical parts from the image leaving you with only the rooms you haven't explored. However you now have no reference to find said rooms.