Here are some tips about the installation of the shower door. First you need to measure the width of the shower’s opening that should be done on at least three places: threshold, up to 5 feet, and halfway in between. If the wall is more than ½ inch out of vertical over 5 feet than there you will need a matching filler strip so that the door can be made plumb. Be careful where the door pivot is and  measure the distance to bath fixtures or any other potential obstacles that door might hit.
Draw a sketch of the shower in plain view (looking down from above) and in elevation (side view). Fill in all the measurements and take it with you to the showroom. Than the process of installation of the door may start. Follow the full guidance in the link bellow.

Source.

...