Fill a bucket with room temperature water.
Bring the bucket with you to the hydrangea bush.
Cut the stems in the cool part of the day. The early morning works well.
Put the stems immediately into the bucket as soon you've cut them. This prevents air bubbles from entering the stem and preventing water from reaching the flower.
Bring the bucket and blooms into the house. Boil water. Pour an inch of boiling water into a container. Cut the stems of the hydrangea to the length needed for the vase or arrangement. Put the stems in the slightly cooled water for 30 seconds, holding them upright. This removes any sap that may block the stem's ability to take up water. Remove and immediately place in tepid water in the vase or arrangement.