1.
Copy the maps folder over to a folder in your Black Ops 2 directory.
There should be 2 folders called t6r, and then another folder called data.
1A.
If you don't have any of those folders, create a new folder using CTRL + SHIFT + N and call it (without quotation marks) "t6r". Then enter "t6r" and create another folder called "data".
Your root directory should look like this:
Call of Duty: Black Ops 2 > t6r > data
2.
Open the ZIP folder and go to the folder titled "PC GSC."
3.
Copy the "maps" folder over to that "data" folder you've created (or if it existed) earlier.
For the most part, that should be entirely it. One thing I will note is that this mod menu might not work on Origins. But I know it should for any other map.
Expect green text when you load into a game. This is the mod menu telling you that the installation must have been successful.
Hopefully that was not too complicated.