Check the threshold of the door frame to ensure that it is level. Use a level. If the threshold isn't level, nail shims to the threshold until you get a level reading.
Place a strip of adhesive-backed aluminum tape across the threshold so that it covers the front edge of the threshold and the front part of the top edge of the threshold. Use a piece of tape long enough to cover the bottom 2 inches of the side jambs, as well as the threshold. Make a small cut in the tape at the corners where the threshold and jambs meet so that you can press the tape flat against the surface to prevent leaks.
Place a second strip of aluminum tape on the threshold and jambs behind the first strip, overlapping the edges of the first strip.
Apply two thick, zigzag beads of silicone adhesive on top of the threshold. This helps to provide a watertight barrier underneath the French doors.
Stand on the patio, and with the help of an assistant, place the bottom of the French door frame against the edge of the threshold, with the top of the door frame leaning out toward you and your helper. Push the top of the frame into the doorway, and hammer nails partway into the corners of the flange around the top of the door frame to temporarily hold it in place.
Check the frame of the doors for square by measuring the distance diagonally across the doors in both directions. If the measurements are different, slide shims between the doors and the door frame inside the wall, and adjust them until the measurements are even.
Secure the doors to the wall by nailing the flange every 6 to 8 inches around the doors.
Apply silicone adhesive to the header trim above the doors. Place the drip cap onto the header trim, pressing it into the adhesive. Then nail the back edge to the wall above the doors.
Place silicone caulk along the edges of the door frame where it meets the exterior wall to seal the joint.
Apply spray foam insulation between the wall frame and the doors on the interior of the house.
Install the interior casing onto the door frame.