Many of us will find that the moldings on the bottom of our doors are starting to fall off. Unfortunately, there is no way to fix them once they start to rot. The only option is to replace them. This is a common enough problem that most Audi dealers in salt belt areas tend to stock them.
Step one is to remove the old molding if it's still on the car. no need to be gentle, just pull, it will probably come right off. What's under it are metal posts, which are part of the door skin, and the plastic retaining bars. These need to stay on the door. The very front edge of the molding is held on by a screw, to remove it, pull all remaining molding away from the door, leaving behind the metal nut. Use locking pliers to hold it while you remove the screw.
Next, line up the new molding strip with the plastic retaining bars. Push it on, and slide it forward till the rear end of the new molding is even with the edge of the door. Look at the hole where the screw came out, make sure that the threaded nut is visible through the hole. Put the screw back in, and you are done!