The most important step in installing a shower door is to measure carefully
. Doors need to be accurately measured within 1/16 of an inch so there isn't much wiggle room for mistakes. The most common mistake made by homeowners and even contractors in the installation process shows up here in the measuring steps. It is vital to not measure before the tile or marble is in place because it will create erroneous measurements to measure too soon.
The next step is to plumb the walls. If the walls are off plumb you will need to figure out where the glass door needs to be cut larger to compensate in order to have a tight fitting and water proof door.
The next step is to make sure your shower door has enough clearance to open correctly. Building regulations require that the door swing out to open so before you install make sure that the door can do that without banging into a wall, toilet or sink in the process.
Since there is no standard height for doors you will need to measure and decided how tall you want the door to be. It should at least clear the showerhead to avoid messy shower overspray and have an aesthetic look.
The next step is the actual installation of the door itself. Depending on the style of door and shower enclosure the actual steps will vary from project to project but the basics will include verifying the door will fit the opening, installing hinges and door handles and putting the sweep in place. Shower doors are designed to not fall off their hinges once the hinges are in place for safety reasons so once you get the hinges in place you are safely able to maneuver the door around. Start with the middle hinge so adjustments can more easily be made on the upper and lower hinges as needed to adjust the fit.
When positioning the door use a wooden shim to give the door 3/8" clearance from the bottom so the sweep can be put in place. Align the door in place and make sure it is level before you mark the location where you will need to drill holes to place the door. If you are drilling through tile or marble use a 1/4" masonry bit and work slowly to avoid damaging the tile or marble. When you start putting in the screws remember to start with the top outside and then the bottom outside. The final step is to install the sweep, which should extend 1/2" beyond the door width on both sides.
If you did everything correctly you should have a shower door that opens smoothly without getting stuck or banging into fixtures and is waterproof. And that's all there is to it.