Cut a space 2 inches wider than the door in the bottom plate of the wall frame using a reciprocating saw. Remove any studs from the wall frame in the space where the door will be.
Cut two pieces of lumber as long as the space between the bottom plate and the top plate. These are the king studs. Nail the king studs into place, one on each side of the opening you cut for the door. Each king stud should be set 1 1/2 inches away from the cut you made in the bottom plate. Use metal connector brackets to nail the king studs at perfect 90-degree angles to the bottom and top plates.
Cut the jack studs. These studs are 1 inch taller than the door. Nail one jack stud to each king stud so the edge of the jack stud is flush with the cut you made in the bottom plate for the door.
Cut a piece of wood sized to fit between the king studs on top of the jack studs. This piece is called the header. If this wall were load bearing, the header would be sized according to building codes. Since this wall is not load bearing, a standard two-by-four will do. Nail the header into place, resting above the jack studs, between the king studs. Use metal connector brackets so the header fits at a perfect 90-degree angle to the king studs.
Cut two pieces of lumber sized to fit between the header and top plate. These are called the cripple studs. Nail two cripple studs above the door, spaced 12 inches apart. Use metal connector brackets.