It is easy. Depending on where you live, the township will tell you what back flow device they will allow. Here in Canada (Toronto) you have to use a dual check style. You determine the size of back flow prevent-or, by the diameter of your main water feed. You can also determine the size of recovery tank needed to contain the water, when the back flow prevent-or dumps, or does its job. It all has to be piped, so that it can be bypassed during a repair. The city you live in should tell you what is required. Putting it in is not so bad. It usually would go where your water heater is, or where your water feed enters the house.
