How to Frame a Door Opening: 13 Steps (with Pictures) - wikiHow
To make the studs, measure the height of the door to be installed. Cut 2x4s to the height of the door plus 1 1/2 inches (5.1 cm) to allow room for the top of the frame and allowance for leveling the jamb. To make the door header, cut 2x4s to the width of the original opening. A "king stud" is one that runs continuously from the top plate ...