Home improvement stores such as The Home Depot and Menards sell decorative door inserts in store, in common sizes.
Door inserts can be installed either by a professional, or using detailed instructions while the door is still on the hinges. Using a jigsaw, cut an opening for the insert as per the specific instructions included with the product. Install the trim kit on the exterior side, then insert the glass from the interior side, and finish with the second trim kit. Fasten the trim kit together with screws to complete the installation.Learn more about Doors, Windows, & Locks