Tear off the upper edge of the cup after the water becomes ice. This will expose some of the ice and provide an insulated grip for the ice massaging. Massage the injured area for 5 to 10 minute intervals. Do this on and off for a couple of hours. Do not apply ice any longer than this.
Some precautions are necessary when treating an injury with ice.
Do not leave ice directly on the skin. Keep it moving or wrap it in a wet towel. To avoid skin damage, stop icing once the skin is numb. Be careful with commercial cold packs. They may be colder than regular ice.