Set a level against the face of the right-hand king stud. Adjust the bottom of the stud in or out as needed to center the bubble in the level. Repeat this same leveling procedure on the king stud to the left of the opening. Measure the distance between the two king, or outside studs, of the door opening in the wall frame. Cut two pieces of 2-by-6 to fit between them, using a circular saw. Cut a piece of ½-inch plywood to the same width and length as the 2-by-6 you just cut..
Spread wood glue on both sides of the plywood. Sandwich the ½-inch plywood between the two pieces of 2-by-6. Drive 3-inch treated deck screws through the top 2-by-6 and the plywood into the bottom 2-by-6 with a drill. Position them in two rows with screws every 6 inches. This is your header.
Measure up from the floor to 82 inches, using a measuring tape, and mark each king stud at that point with a pencil. Hammer the header into position between the king studs. Use a level held against the bottom of the header to make sure it is level. Tap the header up or down at one side to center the bubble as needed. Check the two faces of the header to make sure they are flush with the edges of the king studs. Drive 16-penny nails at an angle through the face of the header into the king studs.
Cut two studs to 82 inches with the circular saw. These are the jack studs. Place them underneath the header on either side of the opening and hammer into place against the face of the king studs. Nail through the faces of the jack studs and into the king studs with 16-penny nails.