Hi, the process for creating levels is exactly the same as what is described for the first level. I determine where my second level will be in relationship to the first level for obvious structural considerations and connection to vertical circulation. Each level is grouped separately.
All floors are kept on separate layers and all the groups that compose the OB or "Objects" such as floors, walls, windows, doors, cabinets etc are grouped together on the LO or "Level" layer. This allows you to easily toggle on and off entire levels with activating a single layer. The SP layer is for special items that either create vector line work in Layout or provide information in the model - like area calculations.
The key is understanding the hierarchy of the layers and how the system works. This is discussed in detail in the book and is key for quickly preparing scenes in Layout.
I hope that helps.