Step 1
Select the door that will have the astragal mounted on it. Choose the inactive door, which can be the door that either does not open at all or is not the door that is primarily used. Identify which side of the door the main piece of the astragal will appear on -- it should be on the side of the French doors where the primary door swings away from you to open it.
Step 2
Measure the height of the inside of the inactive door, the space between the two doors when closed and the thickness of the door end. Purchase an astragal kit to these specifications, as astragal strips can be customized to fit many door heights and thicknesses. An improperly fitting astragal may interfere with how the doors open and close and may not seal the crack in between them.
Step 3
Open the doors so you have complete access to the inactive door. Check that the molding part of the astragal is on the proper side of the door. Hold the astragal strip against the door end and press. The astragal strip will flex open and clamp onto the end of the door. Press all the way down the strip until it is completely on.
Step 4
Locate the screw holes and drive the kit screws into the door. Screw them in until the tops are flush with the astragal strip to avoid interference with opening and closing the doors.
Step 5
Slide the plastic insert strip into the channels of the astragal to cover up the screws and provide a clean-looking finish. Test the doors a few times to ensure a good fit and to verify that there are no gaps between the doors.