From Halopedia, the Halo wiki
First, you have to have at least two of the objects that you want to ghost forge. Next, go into forge mode. Press up on the D-Pad and X at the same time. If done correctly, you will have the item list while in player mode. This may take a few tries.
Once in the item list and in player mode, go to the object you want to ghost forge and press A. Repeat for the same object so that you will have two of the same objects on the field.
Go to one of the objects and set its respawn time to never. Then grab the other object and wait for the first object to disappear. Once the object that was set to never disappears, then press X and set the runtime minimum to the runtime maximum.
Move away, and you will notice that you can move the object through the ground and other objects, it can also be placed in air and will stay there.
The final thing to do is to start a new round and delete the object you set to never. It will be where you spawned it at the beginning in the air.
- This can be used to make great structures and racetracks with ease due to the fact you can place the object in air and in real time with other objects
- The objects can only go halfway into the ground due to the game's code for placing objects. This is because the center of the object acts as the new "barrier" for the object.