Soak a block of floral foam in water for about five minutes or until completely saturated. Set it on a dish rack, to allow the excess water to drain from the foam. Then, set it on a footed cake plate.
Cut the calla lilies to four our five inches in length, depending in the height requirements of the arrangement. Be sure to cut the ends of the stems at a 45 degree angle, so the flowers will soak up more water, which will help to keep them healthier-looking for a longer period of time.
Cover the soaked flower foam with Spanish moss, to hide its foam appearance. If necessary, use floral pins to attach the moss to the foam; you can remove these once the flowers and greenery are inserted.
Insert the cut calla lilies into the soaked foam, directly over the moss. Arrange with taller calla lilies in the center and gradually-shorter calla lilies at the edges.
Add some ferns, ivy or other greenery between the calla lilies, to fill in any bare spots which might remain. Finish the piece, by inserting baby's breath around and between the flowers and greenery.