If you do not have an ice cream maker, follow these instructions. You can use your favorite ice cream recipe with this method. I have done with my Sunday School class for the last month and they are constantly amazed by it. ;)
Place 2 cups of chilled ice cream mixture into a heavy duty quart-sized bag and zip it up securely.
Place the quart bag inside a heavy-duty gallon-sized bag.
Fill the gallon bag with enough crushed ice so that the ice cream bag is completely surrounded.
Add rock salt/ice cream salt and seal up the gallon bag.