The plastic bottom piece to your shower door is commonly referred to as a shower door sweep. If you take a shower and notice that there is water on the floor outside of the shower this is most likely caused by an old or damaged shower door sweep. The shower door sweep keeps water from leaking under the shower door and can be easily replaced after following a few simple steps.
Open the shower door and leave it propped open.
Grasp the outside corner of the door sweep and pull it off the track.
Take the old door sweep to your local hardware store to make sure that you get the same sized door sweep.
Cut the new door sweep to the same length as the old door sweep.
Apply petroleum jelly to the inside edged of the new door sweep so it will easily slide back onto the shower door track.
Run water from the shower onto the new door sweep to make sure it works properly.