For heat pack: Simply just activate it. After some time it will become ruined and still remain hot and will still warm the player carrying it.
For Cooking pot: Put the pot on a heat source and remove it once its temperature reaches 200C. It will remain at 200C indefinitely.
For batteries and charger: Put an empty car battery a powered charger and begin charging. After the battery becomes fully charged, both the charger and battery will be at 1C.