Instructions
Insert the "Halo 3: ODST" multi-player disc in your Xbox 360 and wait for it to load.
Highlight "Forge" on the main menu and press the "A" button.
Highlight "Start Forge" and press "A" again.
Press "Up" on the d-pad to go into edit mode. Press the "X" button to open the menu and press the "Right Bumper" three times to get to the scenery.
Scroll down to the "Container" object and press the "A" button. Move the container around with the right analog stick to position it, and press the "A" button to place it in the game world.
Move the left analog stick to move your character around. Fly up until you have a top-down view of the container. Move your cursor with the right analog stick so that it is not aimed at the container (otherwise the container will be selected) and press "X."
Select the object that you want to float, move it on top of the container and press the "A" button to place it. Repeat this process for all the objects you want to float.
Move your cursor over the container so that it turns green with a hand icon. Press the "Y" button to delete the container. All the objects that are on top of the container will remain floating.
Press "Up" on the d-pad to leave edit mode. All the objects will remain floating.