Well the first step is to determine what local ordinances you need to comply with.
The second is to decide if you're going to comply with those ordinances, or go a cheaper route, or the cheapest route.
So you would call your local building department and ask what's the cheapest back flow prevention device that will satisfy the local building code. If you call as yourself, this could trigger them wanting to take a look, or making you obtain a permit or subjecting your project to an inspection afterwards. That would pretty much decide for you what you'll be using in terms of the device, and how it must be plumbed. The downside of that will be cost, the upside is that you might be able to get an inspector out to your property for a "pre-inspection," during which he will describe what you need to do and how you need to do it. I used this process on a plumbing project I didn't quite know how to do, and my city sent out a guy and he walked me through it. He actually appreciated the opportunity for us both to be on the same page. YMMV.
Now if you don't want your local building department involved, you'll need to call anonymously to get the information, which they may or may not want to provide anonymously, because they might know what you're up to.
Or you could try and look up the local codes yourself and determine what you need to do. You could possibly reference the code here, and we'll help you sort through it.
The above process somewhat justifies what a plumber might charge. It's not just time and materials, after all, it's their know how and experience, including their know how about local codes and how to deal with local authorities.