Measure and mark the width of the door frame on your sill plate, the bottom, horizontal framing element on which the wall studs will stand. Mark the width with the carpenter's pencil. Mark the sill plate every 16 inches on each side of the door marks.
Cut two 2-by-4 studs to 80 3/4-inches and toenail -- drive a nail at a 45-degree angle through the stud and sole plate -- one of them to the sill plate at each door mark. These studs, to which the door frame will attach directly, are known as triggers.
Toenail a standard length 2-by-4 next to each trigger, outside the door frame marks. These studs are known as king studs.
Cut a 2-by-12-inch header 5 inches wider than the door frame and place it atop the triggers and between the king studs. Nail it into place by nailing through the king studs.
Toenail a 2-by-4 stud to the sill plate at each 16-inch mark.
Lay another 2-by-4 at the end of the studs opposite the sill plate. This is your top plate. Toenail the studs to the top plate.
Cut and remove the sill plate between the door marks with your circular saw. Insert the door frame and nail it to the triggers with finish nails.