Measure the full length, width and thickness of both your old door and the frame, using a measuring tape. Purchase a replacement door that is exactly the same size as your old door. Measure the distance from the top of your old door to the hinges, and mark the exact place where each hinge should go on the new door.
Lay a tarp down underneath your old door to catch debris that may fall when you remove the door. The tarp will also catch broken glass if you accidentally break it while you are removing the door.
Unscrew the hinges from the old door, using a screwdriver or drill. Remove the door, carrying it carefully so as not to break the glass.
Line up your new door with the existing hinges. Attach the hinges to the door by driving screws through the hinges and into the door itself, securing it in place.