Game_Mod is a modification to Black Ops 1 which allows you to load and play custom maps just like World at War.
This tutorial requires you to download the latest version of Game_Mod from Game_Mod Releases. When you are on the page, download the zip archive.
To install Game_Mod navigate to your Black Ops 1 install directory "steamapps\common\Call of Duty Black Ops". Unzip and copy the files over to this path, preserving the layout of the zip. If you are updating and are prompted to overwrite files, just say yes.
Game_Mod will not run when you launch the game normally, this is done to allow you to enable or disable it safely. To launch Game_Mod, you can execute the file BO_Mods.bat that came with the download, this file is in the root of your Black Ops 1 install directory.
Only continue here if your installation requires this. Head to the root Black Ops 1 folder, then find the file bin\launcher_ldr.exe this file is the one that runs Game_Mod properly. To make it require administrator access, right click and go to properties then security tab, at the bottom you will see Run this program as administrator: