Cut the top off of an empty milk carton and rinse thoroughly with water. Cut the carton to the height you would like your centerpiece to be.
Arrange flowers and fresh greens in the carton. If you would like your centerpiece to include a candle, simply place a large pillar candle in the center of the carton then place flowers and fresh greens around it. Make sure the wick of the candle extends above the edge of the carton.
Fill the carton with water and carefully place it in the freezer.
Remove the carton from the freezer when the ice is solid. Dip it in a bowl of warm water for a few seconds and peel the paper away.
Place the sculpture back in the freezer until just before serving time.
Remove the sculpture and place it on a tray or in a shallow bowl in the center of the table. Surround the sculpture with more flowers and greens.