How to Create a Doorway
By holding [R] while placing a wall, you can bring up the variant menu. One of the options you’ll see is “Door Frame” — select it with your mouse, and you should then be able to place a wall with a space for a door in it.
How to Create a Window
Spaces for windows are created the same way doorways are: hold [R] while placing a wall to bring up the variant menu. One of the options you’ll see is “Window Frame” — select it with your mouse, and you should then be able to place a wall with a space for a window in it.