Hold your curtain rail into position against your fiber glass door.
Use a bubble level to ensure the rail is level, and mark where the screw holes need to go on your door's surface.
Insert into your drill a drill bit slightly smaller than the width of the screws you're using. Then, measure the length of your screws and set the depth control on your drill. Make sure the screws you're using won't go through to the other side of the door.
Drill screw holes into the door.
Screw your curtain rail into place, and put up your curtains.