Identify which of the doors is active and which is the stationary door. If only one door will open regularly, that is the active door. Active doors generally have the doorknob lock while stationary doors have a floor-locking mechanism in place. If you cannot tell, choose the door that is used most as the active door.
Open and close the active door to see which way it swings. Stand on the side of the active door where you have to push the door away from you to open it. The astragal will be installed on the inactive door, and the molding piece will be on the side of the door you're standing on when the doors swing away from you.
Measure the height of the French door from the top of the threshold to just under the door frame. Record that measurement. Measure the space between the two doors and record that measurement. You'll also need to record the thickness of the door. Purchase an astragal that fits the specifications of the door from a door and window supply business that carries a variety of astragals. Many companies will do custom cuts for height.
Clean the surfaces of both doors with an all-purpose cleaner and a cleaning rag to eliminate any dirt and debris that might be stuck there.
Press the astragal strip onto the side of the door so that it wraps around. Begin at the top and press the astragal onto the door, working your way down to the bottom.
Locate the screw holes in the astragal strip. Using a drill, create pilot holes. Screw the astragal to the door until the heads of the screws are flush with the door surface. If the astragal kit came with channel covers, snap them in place over the screws.