Clean both sides of the glass insert panel with a window cleaner and paper towels.
Loosen and remove the hinges that secure the old door to the closet with a screwdriver. Keep the screws aside, since you need to use these when reinstalling the door.
Lay the door face down on a clean, level surface. Center and lower the glass insert in the closet door. Leave a 1/16-inch gap all around the glass and closet door step. Remove the glass panel from the door and set it aside.
Measure and cut lengths of painter’s tape and install these around the edges of the glass insert. The tape protects the glass from excess caulk. Also apply painter’s tape around the door step opening, ensuring the door faces downward.
Load a clear silicone cartridge in a caulking gun. Puncture the tube’s seal using the tool on the gun. Pump the gun until the silicone starts to flow. Wipe off excess silicone from the nozzle using a clean paper towel.
Insert a tiny bead of silicone into the corners of the closet door step. Center and lower the glass insert into the closet frame, lowering one side at a time.
Run a thin bead of silicone caulk between the cabinet door and glass insert. Dip your thumb into a bowl of water and run it along the silicone bead until smooth. Immediately remove the strips of painter’s tape to keep the silicone caulk from drying into it. Allow the caulk to cure for the period of time specified on its label.
Hang the door in its spot on the location, using a screwdriver to drive mounting screws into the hinges.