There's no actual way to get second floor in game without doing it manually.
But here's how I did it.
So if you look at the stairs I used, its actually the box option in the design area you can use. Then after I made the stairs, I used the rugs to make the floor for the second floor part of the build. afterwards I used the pink ombre poster board to create the inside walls of the second floor and then I decorated the inside of the second floor, which you can do that any way you want.
After I finished the decoration, I made the outside walls by using the mirrors.
Every item used can be found in the side bar filled with possible items to use.
Hope this helped ദ്ദി(｡•̀ ,<)~✩‧₊