Run water in the kitchen sink. Hold the stems of two to three carnations under the running water. The running water keeps air bubbles from forming. Air bubbles will stop the carnations from absorbing water and nutrients, thus shorten their lifespan.
Cut the stems with a sharp pair of scissors. Position the scissors directly above a node on the stem and at an angle. Cut the stem and set it to the side as you cut the remaining stems.
Fill a vase 2 to 3 inches from the top with warm water. Add a flower preservative to the water. Follow the directions on the flower preservative container for how much to add to the water in the vase.
Place the carnations one flower at a time in the vase. Set the vase in a cool spot within the home, out of direct sunlight and away from any ovens, heaters or furnace registers that could reduce the life of the carnations.