Fill a vase or other waterproof container one-half full with fresh water. Hydrangea flowers and calla lilies are rather bulky and heavy so a vase that is wide and low with a wide neck opening is better than a vase with a narrow, small neck and base.
Remove any leaves on the lower stems of the hydrangea flower heads. Strip them off by hand or snip them off with florist pruners, scissors or a knife. You don't want stem leaves to submerge in the water in the vase; that causes them to quickly rot and foul the water.
Cut the hydrangea stems to a length appropriate to fit in the mouth of the vase. Fill the vase, one stem at a time, to create a loose massed cluster of only green hydrangeas.
Insert calla lilies into the mass of green hydrangea flower heads. Gauge the length of the calla lily stem before slicing it at an angle with a sharp knife. Avoid crushing the stem as you cut it. The calla lily's trumpet-like bloom should rest at the same height or slightly higher than the surrounding stems and petals of the green hydrangeas. Calla flowers and stems bruise easily, so try to limit touching the flower as much as possible.
Slide the cut calla lily stems through the matrix of hydrangea heads into the water at the base of the vase. Space additional calla lilies evenly across the entire bouquet of hydrangeas.