1. Do this outside. The condensation on the bag is considerable and makes a big mess. If there is any leak of the actual water from inside the bag, you will have really salty water dripped all over your house (do not ask me how I know this).
2. Shake gently. If you are too rough, the ice cream bag will get salt water into it and muck up the taste. So, no need to throw the bag all over the yard. Just shake back and forth or roll it around.
3. Make sure you surround the ice cream bag with the ice. If it is just sitting on top, it will not freeze.
4. Do not skimp on salt. It makes the temperature colder than the ice and is the magic that freezes the ice cream mixture.