Step One: Purchase a bottle of rubbing alcohol and a box of gallon size freezer bags.
Step Two: Pour the alcohol in the bag, then, using the same container the alcohol was in, fill the container with water and pour it in the same bag. You will do this twice. So that is 2 parts water to 1 part alcohol.
Step Three: Seal bag and place it in another Ziploc bag - SEAL that bag
Step Four: Lay flat in freezer and freeze for 3 hours.
The alcohol doesn't allow the mixture to freeze all the way - so you get a nice slushy mix! When it starts to loose it's icy freshness - just refreeze! :)
According to WebMD... Always keep a cloth between your skin and the ice pack, and press firmly against all the curves of the affected area. Do not apply ice for longer than 15 to 20 minutes at a time, and do not fall asleep with the ice on your skin.