Set the new threshold on a piece of 1/4-inch hardboard. Line up one of the threshold's long sides with the hardboard's side. Trace along the threshold's other long side with a pencil. Cut the hardboard to the same width as the threshold with a jigsaw to form a template. Cut the template to approximately 6 inches long.
Position the template against one of the Andersen french door's jambs, in the same position the threshold will be mounted. Hold a scribe tool above the widest gap between the template and side jamb. Open the scribe tool to span the distance, then trace the tool along the Andersen french door's jamb to mark the contour on the template.
Cut the template along the scribe lines. Test the template against the side jamb and file it with a saw rasp as necessary until it fits tight.
Measure the french door opening's width and add 1/16 inch. Transfer this measurement to the new threshold and cut it straight across with a jigsaw. Lay the template face down on a flat work surface.
Lay the template on one of the new threshold's ends. Trace along the template to transfer the Andersen french door's jamb contour to the threshold. Flip the template over to create a mirror image and lay it on the threshold's other end. Trace around the template to transfer the contours to this end.
Cut the new threshold along the contours with a jigsaw. Test the threshold in the door opening and file it with a saw rasp as necessary until both ends fit tight against the door jambs. Close the Andersen french doors and check for rubbing.
Lay the new threshold face down. Place a tube of silicone caulk in a caulk gun and remove the tip with a utility knife. Squeeze two beads of caulk lengthwise down the threshold. Fit the threshold back into place in the door opening.
Drill four pilot holes through the threshold with a drill and 1/8-inch drill bit. Drive a screw -- either 1/2-inch or 1-inch depending on the threshold's thickness -- through each pilot hole.