From the control panel, you will access Properties / My properties.
You will see a list with all your properties. You will click on the orange Edit button that is in the right column of the property in which you want to add rooms.
First of all, the Rooms tab may be disabled. To activate it you must go to options and activate it from there.
Once this is done, you can access the Rooms tab and add as many rooms as you want with the orange button + Add room.