How do I build a second floor.

I see all these videos of people building second floors, or adding walls on top of walls, and I try to do it myself, and I've tried like everything, but it will not let me put a wall on top of a wall. First I tried to build half walls into a square, and then tried to add another half wall on top of them, but it did not work. I also tried to put an actual room on top, and it still did not let me. Then I tried an actual room, and put a room on top of that, and it still didn't work, and it also tried to put half walls on top of the room, and it did not work. So I'm not sure what to do.