Decorative door inserts do not require a custom door. Manufacturers of decorative door inserts specify measurements to ensure a good fit for the door in question.
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.