How to winterize a sprinkler system back flow preventer

Questions this video answers:

1. How do I winterize my sprinkler system?
2. What is a backflow preventer for a sprinkler system?
3. Why do I need to winterize the backflow preventer?
4. How do I know if the water valve is on or off?
5. What steps should I take to winterize the backflow preventer?
6. What are the additional measures I can take for extra protection against freezing?