Cut the cardboard of your box sides as the heavy black lines indicate. Fold outward where there are dotted lines.
A. Single door. Cut at top and side (if need be, at the base also). Fold the cardboard outward to make a door.
B. Double door. Cut the square at top and down its center. (If need be, cut the base of the square also.) Fold both sections outward.
C. Door with window in it. Cut out a square from the single door. Cut the door at top and side. Fold it outward.