Place the milk, sugar and vanilla into the doubled-up medium zip-seal bags. Close and secure with sticky tape if the seal isn’t strong. Tip the ice and salt into the large bag, then place the smaller bags inside the larger one, and seal well. You will need to keep the bags moving for 10-15 mins to freeze the milk.When adding mix-ins like vanilla, almond, chocolate chips or any other make sure you do so only when the batter cools down and add them in the end just before churning.