Instructions
Enter the "Forge Lobby." The Forge Lobby can be entered from the main menu or by using the "switch lobby" option while in any other lobby.
Select the map you wish to edit using forge. Once you select which map you wish to edit, the map will load as if you are playing the game normally.
Press the "up" button on the d-pad to enter "edit" mode. Once you are in edit mode, you will no longer be your Spartan character, you will now be a free-floating "monitor" that can manipulate nearly every object in the game.
Press the "start" button to view the control layout for the monitor. Once you are familiar with the controls, you can simply start moving, augmenting, deleting and adding objects as you see fit.
Press the "up" button when you are finished editing to become your Spartan character again. Now you can test your newly augmented map. Save it by pressing "start" and selecting "save map as..."