Place two sawhorses at a level location and lay the door on the sawhorses.
Mark the area on the door where you will install the window. With a pencil, make all four sides of the hole 1/8 inch wider than the glass panel you are using for the window. The gap will allow the glass to expand during the various seasons.
Secure a 1/2-inch drill bit to the power drill. Drill a small hole on the inside of each outline corner. Place the jigsaw with a fine-toothed blade through one hole. Don safety gear and gloves, then use the jigsaw to cut along the outline from one corner or another. Continue in this manner until you have cut the entire hole.
Smooth the cut with sandpaper. Paint bare wood that creating the hole has exposed with a paint that matches the door. This will help prevent possible wood rot. Let the paint dry for several hours.
Add a beaded line of acrylic siliconized caulking around the hole. Set the glass in position and use a hammer and penny nails to secure the precut molding around the glass and hole.