Fill a small resealable bag with 1/2 cup milk or half-and-half, 1 tablespoon sugar, and 1/4 teaspoon vanilla
Seal the small bag tightly, removing as much air as possible
Fill a larger resealable bag halfway with ice
Add 1/2 cup salt to the ice in the larger bag
Place the sealed small bag inside the larger bag
Seal the larger bag tightly
Shake the bag vigorously for 5 to 10 minutes
Wear gloves or wrap the bag in a towel while shaking
Check the small bag for thickened ice cream
Remove the small bag from the larger bag
Rinse the outside of the small bag with cold water
Open the small bag and eat the ice cream immediately