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There called a Quick Coupler along with a Quick Coupler Key

Install right after your back flow assembly / irrigation shut off

Too vague a question - if the system needs blown out, the location of the blowout valve depends on how the supply plumbing is laid out.

Well.. Yes people do blow from the back flow asseembly.. but it can damage them so it's not recommended..

Quoted from "2Quik"

So the best and safest way is to install a Quick Coupling Valve and right after my main shut off and filter?...

Building codes require the connection point to be AFTER a backflow preventer. That's one of the purposes of a backflow preventer. Among other things, the backflow preventer ensures that pressurized air (along with oils and any other "stuff" found in an air compressor) don't get pushed back into the potable water supply when someone forgets to close the shutoff valve between the backflow preventer and the connection point.

So what you need (in this order) is:
1. Main shutoff valve - to totally isolate the entire irrigation system from the rest of the water supply.
2. Filter - so that the water is filtered before it gets to the backflow preventer.
3. Backflow preventer - to protect any potable water linked to the irrigation system.
4. Another shutoff valve - needed to isolate the backflow preventer for blowouts for winterization.
5. Connection point for blowouts - doesn't have to be a quick-connect, but need some means of connection for blowout.
6. Irrigation valves.

Now you might not need a seperate shutoff between the backflow and blowout, if the backflow preventer already has shutoff valves at each end of it. Then there's the issue of how to winterize the backflow preventer itself. What would seem easiest would be to attach the backflow preventer with a pair of unions so that it can be simply removed from the system. If you go this route, I would suggest you buy an additional union and attach dead-end (i.e. capped off) pieces of pipe to the two union halfs. The idea is that when you remove the backflow preventer for the winter, you can protect the pipes from creapy crawlers by attatching the capped off pieces of pipe to the two union halves from where the backflow preventer was removed.

All this is speculation - to be more exact, the install location is required, along with the location of the system connection and what type of backflow prevention is being used.

Yeah that works too, just install a shut off prior to the adapaters

When I winterize .. Lots of system don't have quick couplers.

I do a quick 1 second burst of air through the assembly with all test cocks open, then blow through the last tc because this is past the check

Filter on a city supplied water irrigation system? A little over kill and its not really needed. go for it if you want though.

As boots vaquely said. You need to check with your local city codes on proper installment and the proper back flow assembly to be installed.