Place ice cubes and salt in a quart-sized Ziploc bag.
Place juice in a sandwich-sized Ziploc bag, press out all the air and seal tightly. If it isn't sealed tightly enough, salt will get in and it won't taste good.
Place the small bag of juice IN the larger bag of ice and seal. The larger bag will be holding the ice, salt, and the baggie of juice. *PLEASE NOTE: You will have two separate bags - a quart bag with ice and salt, and a sandwich bag with juice.
Shake for about 3-5 minutes or until juice gets slushy. If your hands get too cold, wrap a dish cloth around the bag or use gloves and continue shaking.
If your slushy isn't getting slushified. Add more ice and salt to the outer bag and keep shaking.
Pour slush into a cup and serve immediately.