Remove leaves from the hydrangeas by clipping them off near the stem. Cut floral foam to fit in the bottom of a vase or other container for the arrangement. Push the floral foam into the bottom of the vase so it fits tightly. This will hold the flowers in place.
Create a dome arrangement by cutting three hydrangea stems the same length, twice the height of the container or less. Place those hydrangeas at the center of the container, inserting each stem gently 2 inches into the foam. Cut the stems of the next group of hydrangeas 2 to 3 inches shorter than the central hydrangeas, and insert them in a circle around the central hydrangeas. The number you need depends on the size of the hydrangea blooms. Use enough to surround the first three hydrangeas without crowding the blooms. For a larger display, cut another group of hydrangeas 2 to 3 inches shorter than the last group, and insert them in a ring all the way around.
Form an unstructured bouquet of hydrangeas by grouping them in your non-dominant hand the same way you'd carry a bouquet. Adjust the blooms so that the flowers don't crowd each other by shifting the stems with your dominant hand so that some flowers extend higher than others in the bouquet. Tie a cord or raffia -- a decorative natural plant material -- around the stems within six inches of the bloom to keep the flowers in position. Clip each stem off straight across so they're even on the bottom.
Arrange hydrangeas with accent flowers. For example, cut dried hydrangeas to fit in a basket for a low mound of color or lacy white background. Add purple statice or rose straw flowers between the hydrangeas to add color and dimension to the arrangement. Cut the stems of the accent flowers 1/2-inch to 1-inch longer than the hydrangea stems to have them hover just above the cloud of hydrangeas.