Put milk, sugar, and vanilla in a small resealable bag
Seal the small bag tightly, pressing out extra air
Fill a large resealable bag with ice
Add salt to the ice in the large bag
Place the small bag inside the large bag
Seal the large bag tightly
Wrap the bag in a towel or wear gloves
Shake and knead the bag for 5 to 10 minutes
Check that the mixture has thickened into ice cream
Remove the small bag from the large bag
Rinse off any saltwater from the outside of the small bag
Open the small bag and serve the ice cream