Instructions
From the menu, choose the neighborhood in which you would like to build your apartment.
Choose an empty lot and choose "Residential" zoning.
Build the apartment as you would a regular house. Set up the walls to your liking and add wallpaper and furnishings.
Press "Ctrl-Shift-C." This will being up the cheat menu.
Type: changelotzoning apartmentbase
Press "Enter."
Save the lot and exit to the neighborhood. Your newly created apartment will be ready to use.