Spray a rag with an all-purpose cleaner. Wipe the edge of the inactive door to remove dirt and debris.
Measure the height of the door frame from the floor up where the inactive door sits with a tape measure and note the measurement. Most double doors come in 84- or 96-inch heights.
Unroll the compression seal and cut it with a utility knife to the same length as the height of the door frame.
Position the compression seal along the edge of the door behind the astragal with the top edge of the seal touching the top of the door frame and the bottom edge of the seal touching the floor.
Hold the seal in place or have a helper hold the seal in place.
Place a 7/8-inch flat-head screw 1/2-inch from the top of the door, centered to the edge of the door or as close to center as possible without blocking the door lock.
Drive the screw through the compression seal into the side edge of the door with a power screwdriver. Pull the seal, so it is taut and drive a 7/8-inch flat screw 1/2-inch from the bottom of the door. Drive 7/8-inch flathead screws through the compression seal into the side edge of the door every 12- to 15-inches.