Position the metal studs that will form the two sides of the rough opening so that the distance between them is 1 inch greater than the exterior dimensions of the door frame. This provides you with space to adjust the door frame and make it perfectly square and level with shims.
Measure up along the studs on both sides of the door opening to a point that is 1 inch higher than the vertical dimension of the exterior of the door frame. Install a horizontal metal header at this point between the interior faces of the metal studs on both sides of the rough opening. Cut the piece for the header long enough so that you have some extra on the ends. A metal stud is shaped like a U. Cut the sides of the U shape off at the ends of the piece of metal and bend the central part down to create a surface that can be screwed to the faces of the studs.
Fill in between the metal header and the top plate by installing studs every 16 horizontal inches.