You made the cabin, the two portal Booleans and the door Boolean. Now you are going to combine them to make the portals and door openings in the cabin:
Select the cabin.
Select the Modifiers button in the header of the Properties window. It's the seventh button from the left, with the little wrench.
In the Modifiers subpanel, press the Add Modifier button with theLMB. Then select Boolean from the pop-up menu.
When the Add Modifier subpanel appears, press the button with the cube, below Object:. Choose Door Boolean from the pop-up menu. Set the Operation: to Difference as shown on the right of the following graphic.
You have just made the door frame.
Press the Add Modifier button with theLMB again. Then select Boolean from the pop-up menu.
When the second Add Modifier subpanel appears, choose Portal Boolean as Object:. Set Operation to Difference as shown on the right of the following screenshot.
You have just made the rear portals...