Slip one end of the header frame in the top of the rough door opening and push the opposite end up into the opening. The frame fits snugly over the drywall and will not fall. It does not require leveling at this time.
Hold the strike jamb of the hollow metal frame at an angle, with the top of the jamb pointing up. The strike jamb is the jamb with the hole for the door strike plate. The top of the frame has two protruding notches that fit into two slots on the end of the header frame.
Push the top of the strike jamb into the slots on the end of the header frame. Push the bottom of the strike jamb into the drywall door opening. Repeat the process for the hinge jamb frame on the opposite side of the door opening.
Place a level across the door opening on the floor. If the floor is level, ensure the bottom of both side frame jambs are resting on the floor. If the floor is not level, place shims under the side frame jamb that requires raising.
Cut a piece of 2-by-4 board to the width of the door with a circular saw. Place the board in between the side frame jambs under the header. This pushes the side frames over the opening jambs enough to accommodate the door.
Slide the board down the length of the opening until it is resting on the floor. Place a level against the hinge side jamb to ensure the jamb is level. Tighten the compression screws on the frame with a Phillips screwdriver. The compression screw heads are on the center of the frame pointing toward you. There are usually five screws along the length of the frame. Repeat for the opposite side.
Install two drywall screws through the strap at the bottom of each side jamb with a cordless screwdriver. The strap is a small flange extending over the drywall.
Push the header frame down over the top of the side jambs, and place a level across the top to ensure it is level. Install self-drilling screws through the corner tabs where the jambs meet the header with a drill and socket adapter. The hollow metal frame is now ready for the door.