Cut your piece of cardboard to 4 inches (10 1/2 centimeters) by 3 1/2 inches (9 centimeters). Now place the cardboard on the foil with the reflective side of the foil down. Then fold it like you see in the pictures.
Step 3: Duct tape
Put duct tape 1/4 inch from the edges of the front of the reflective panel and fold the extra onto the backside. After that cover the back with duct tape.
Step 4: Done!
Now you can pack it in your backpack, emergency, kit etc.