Measure the door panel of your door and decide how big you want the glass part to be.
Choose the type of glass you want to use on your door and have the glass cut to the right size according to the measurements that you took. Leave 1/8 of an inch extra on all the sides of the panel. This is necessary to prevent the glass from breaking due to the seasonal contraction and expansion of the wood.
Remove the door from the hinges and lay it out on a flat surface.
Cut the existing door panel with a circular saw. Cut along the inside edges of the rails that frame each panel. Then cut inside the profiled edge of the rails.
Detach the corners of the panel with a chisel, taking care not to damage the flat surface of the door. Slowly remove the door panel, but do not remove the molding.
Check the sides of the cut area for any damage. Paint the sides with protective wood paint and wait for it to dry completely.
Add a layer of silicone caulk to the inside edge of the area that you cut.
Insert the glass panel into the cut area carefully and press the glass onto the molding that was not removed. Put a new molding on top of the glass panel. The two pieces of wood molding will hold the glass panel in place. Try to choose a new molding that matches the old one.
Nail the molding to the door, taking care not to damage the glass. Use decorative finish nails to fasten the molding onto the door.