10. Cut Any Desired Openings
Allowing traffic flow between multiple containers or installing windows will require you to cut out some openings. A cutting torch or a plasma cutter will make this step relatively simply, but do not make any cuts before you have spoken to an engineer and added reinforcements to your structure.
11. Add Flooring
There is going to be a wooden floor in place already in the bottom of your shipping containers, but you should cover this with epoxy and consider adding an additional subfloor. This is for your physical comfort, and it will also prevent you from being exposed to the pesticides that are often put onto the wooden floor.