Cut the Styrofoam block in 3-inch deep slices with your knife. Hot glue it to the inside of the plastic pot saucer so the foam is within an inch of the edge of the saucer. You may have to use more than one slice to cover the saucer. Cover the foam with a thick layer of Sphagnum moss. Tuck it into the gap between the foam and the edge of the saucer. If it won't stay tucked in, use a dab of hot glue.
Cut the stems of the silk greenery in three different lengths: 4 inches long, 6 inches long and 8 inches long. Arrange the greenery by sticking the stems through the moss into the foam. Start with the 8-inch long stems in the center, then the 6-inch stems and finishing with the 4-inch long stems. Place three stems of trailing ivy or vine at noon, four o'clock and eight o'clock around the saucer.
Cut the stems of the silk flowers in three different lengths: 5 inches, 7 inches and 9 inches. The stems are cut longer than the greenery so the flower heads are held above the greens. Arrange the flowers with the longer stems in the middle, 7-inch stems next and the shortest stems toward the outside of the saucer. Keep the flowers evenly arranged.
Place the saucer with the flower arrangement in the pool. It should float evenly and not list to one side. If it lists, place a glass marble with a dab of hot glue on the opposite side to straighten it out. It might take more than one marble.