Remove the damaged door threshold
First of all, you need to remove the existing threshold installed, in order to make space for the new one. You can cut the existing threshold into two pieces and remove it by using pry bar. Once the old threshold is removed, clean the surface thoroughly, so that you can work properly while installing the new one.
Measure the new threshold
Check the space between the door and the floor. It will allow you to make accurate measurement of the piece of the vinyl bulb door threshold that you need to install. After you are done with the measurement, cut the threshold that you will fit under the door. Place the threshold slightly outward, leaving small space at the corners. It will allow the water to flow away from the threshold. It is really important to place the piece in such a way that it should not look apart from the door when it is closed. Meanwhile, it should not be so close that the water fallen on the door, slips inside the house or the room.
Securing the new threshold
You can now secure the threshold by slightly sliding it under the door. It will tighten the space between the door and the floor. You can now fasten the threshold in place and it will be ready to be used. You will now see a complete difference in the look of your door, as there will not be any space visible under the door. You can also add the same under your bathroom doors, especially when there is chance of water to fall on the door.