Choose the style of window you like best. Your local lumberyard or other window retailers should have a selection readily available, or will order what you have chosen from their catalog of options. Select a style that is narrow enough to fit your door, leaving at least 3 inches on either side.
Remove the door from its hinges by tapping the screwdriver up through the bottom of the hinges, beginning with the bottom hinge and working up. Lay the door on a pair of sawhorses.
Cut out the paper template that comes with the door window kit, using scissors. Position the template on the face of the door. Adjust the template so that it is centered side to side and at the desired height. Use a square to align the template so the window will be level.
Mark around the template on the face of the door. Drill a hole through the face of the door, inside the outline, with a 1/2-inch paddle bit. Insert the blade of a jigsaw into the hole and cut along the outline of the window template. Allow the cutout to drop through onto the floor. Sand the edge of the cutout with 100-grit sandpaper to remove any splinters.
Replace the door on its hinges and tap the pins back into place. Wedge the door open with a door stop, positioned so that both sides of the door are accessible.
Remove the screws from the window kit trim ring and separate the two parts of the window. Insert the outside half of the kit into the window with the glass pane and hold in place. Insert the inside half of the window kit into the inside of the cutout in the door.
Thread the trim ring screws through the holes in the inside half of the window kit into the threaded pockets in the outside half of the window. Tighten the screws with a screwdriver.