A visual walk through (only 30 seconds) can be seen here:
https://youtu.be/FzY9QoJtjrE

Step-by-step walkthrough on text:
1. Click on the Up button next to the house icon on the bottom taskbar near the right.
2. Add a wall like you would normally do, but on the roofing. Make sure to have a room on the 2nd floor where you are going to put the stairs.
3. Go to the first floor (down button to the left of the up button) and add stairs. Make sure the opening hole is within the room you have create upstairs.

Enjoy!