Measure the opening for the French doors. If you're changing the size of a pre-existing opening for the French doors, allow room for the framing. Order the French doors. Delivery of the doors may take 3 to 4 weeks.
Remove any packing material that comes wrapped around the French doors and don't remove any clips from the doors.
Prepare the door opening. Remove the old door. Protect and seal the floor where the doorsill sets by running a line of caulking on the floor.
Center the French doors in the opening. Set it so that it's plumb with the inside wall. Make sure the doors are level on the top and bottom. Temporarily set one hinge jamb with a screw placed about 6 inches below the top.
Use shims as necessary. Place shims on the top as well as both sides of the door unit.
Screw 2 screws in either side of the door just above the top hinges. Check to make sure the door is level again. The gap between the doors and the house should be even all the way around. There shouldn't be any bowed wood. Secure the rest of the door with screws. Unclip the packing clips and make sure both doors operate properly. Install the door hardware.
Place fiberglass insulation in the gap between the house framing and the doors. Install the interior and exterior molding. Caulk the exterior to stop any air leaks between the door frame and the house. Paint the interior moldings and the doors, if necessary.