Measure the height of each of your door panels and cut the 1 inch poly insulation into 2-inch-wide strips about 1 inch shorter than the height of the individual door panels.
Use industrial adhesive to glue the strips onto each door panel. Place one strip near each end of a door panel and two strips near the middle of each door panel. These will act as setbacks or block to keep the next layer, the reflective insulation, from coming in contact with the door surface.
Cut the ½-inch-thick reflective insulation into panels the same size as the garage door panels. Also cut them to fit end to end.
Position the reflective insulation panels onto the back of the door. Depending on the design of your garage door you may be able to just fit them into the sides of the doorframe. Alternatively, you can glue them directly onto the poly strips already in place.
Be sure to install the insulation with the reflective side facing towards the door (not facing into the garage). The reflective surface in combination with the air space you've created will stop that transfer from the door's surface into your garage.