To replace glass panels in a door, remove the glass molding and the broken glass out of the door. Next, measure the door to determine how large the new panels need to be, and cut them out of a large piece of glass. Insert the panels in the door, caulk around the sides of the panels, and attach the old molding to the glass using caulk.
Make sure to keep the molding after you remove it from the old glass using a chisel. If there is any adhesive left on it, remove it with a chisel. When cutting out the glass panels, make sure to leave an extra 1/16 inch on every side of the panel. Use a glass cutter and a straight angle to cut the glass.
Be careful when working with the glass, and wear gloves at all times. Glass is extremely sharp after it's cut out. If you are going to repaint the door after installing the new panels, use painter's tape to protect the glass.
To replace glass panels, buy the replacement panels, find where to dispose of the broken glass, and acquire a glass cutter, caulk, glue, caulking gun and a chisel. Make sure that all of the tools are ready for you to use before starting the replacement.