Boil hot water, pour in water bottle, slip bottle into sock, tuck into the cold spots (lower back, feet, neck, or hands etc. Either in your sleeping bag or if up and around inside your tucked in over shirt or coat pockets. We used to carry small Nalgene bottles that fit in our double boots to thaw them in the morning.

Quart lasts about 6 hours in a sleeping bag for me. I keep my stove handy just outside my shelter to re boil if I get cold again. Works especially well if your clothes or bag is damp. Will dry everything out by morning.