Shower doors come with either sliding or swinging doors. Most sliding shower doors do not use a bottom seal. Outward-swinging shower doors, on the other hand, use a door sweep to seal the door and prevent water from escaping underneath. The door seal may attach with a retainer that screws into the door or may simply slide in and out of a bottom channel. You may need to ask a friend for help in completing this task, but it shouldn't be too strenuous.
Open the shower door and look for screws securing the bottom seal retainer to the door. Remove the screws with a nut driver or Phillips-head screwdriver, and pull the door seal retainer away from the door. If your retainer has no screws, grab the end of the door seal with a pair of needle-nose pliers and pull the seal from the lower channel.
Unroll the new seal and cut it to the length of the old seal with a pair of scissors. If your seal retainer secures with screws, place the old seal against the new seal. Press holes in the new seal with the point of the scissors, using the old seal as a template.
Insert screws through the retainer and position the seal over the screw ends. Install the seal retainer onto the bottom of the door. Tighten the screws.
Slide the new seal into the retainer channel, if your seal slides. Ask a friend to guide the seal at the end while you pull the seal across the channel.
Apply a thin film of petroleum jelly on the seal where it fits into the door channel to help the seal slide easier.