Leave the hydrangeas outdoors until the ground becomes frozen. This gives the plants time to become dormant.
Bring the hydrangea into an unheated area of the house, such as the garage. This will provide enough shelter to keep the hydrangea from freezing, but will not be warm enough to make the plant come out of dormancy early, which can kill it.
Water the hydrangea thoroughly once you have it set in the unheated shelter. Water should come out of the bottom drainage holes. Further watering is not necessary during the winter.
Bring the hydrangea back outside once the last frost passes. Water it again with the same amount of water.