Whether you’re dealing with a six-panel door or a single solid piece of wood, you'll need to cut an insert out of the door. Cutting a panel or two out of the door to insert a stained-glass window involves using a saw to cut out the panels. Similarly, you'll use the same kind of tool to cut out a hole in a door that's a single solid plank of wood. Keep the molding intact on a six-panel door and insert molding when you create the shape yourself. Additionally, if the door consists of a clear window, ensure there's a thermal pane of glass already installed in the window.
When inserting a stained-glass window into a door, consider installing the stained glass directly onto an existing window. You can have a stained-glass window cut to fit the dimensions of the existing window. Clean the clear glass window thoroughly before installing the stained glass. Apply double-sided tape around the perimeter of the glass. Press the fitted stained-glass window in place, holding it there for several seconds to ensure the tape adheres properly. Tape the circumference of the stained-glass window with painter's tape, leaving about 1/8 inch between the edge of the window and the door molding. Apply a thin layer of caulk around the window; use your finger to smooth away excess caulking. Work quickly, as the caulk dries fast. Also, remove the tape before the caulk dries completely.
An additional option is to insert a single window pane inside an open panel. Install the pane alone in the allotted space -- or insert the stained glass between two clear thermal windows. Both options are viable. The clear glass is typically tempered and able to withstand hot and cold temperatures, protecting the stained glass in between. If you choose a solitary pane, know that it will require more delicate care and maintenance.
Although you might find a window to fit your specific needs, you can also have a stained-glass window cut to your specifications. Conversely, if you're artistic, you can make your own stained-glass window. Craft stores may offer classes to teach you how to create a stained-glass masterpiece by adhering pieces of colored glass into leaded frames. Your local window company will have more information on where to find the glass and lead to make a stained-glass window.