Fill a freezer bag with 1 cup of rubbing alcohol (70%) and 2 cups of water.
Try to get as much air out of the freezer bag before sealing it shut.
Place the bag and its contents inside a second freezer bag to contain any leakage
Leave the bag in the freezer for at least an hour.
When it's ready, place a towel between the ice pack and bare skin to avoid burning the skin.