According to William Brown :
If it was well designed, it would include drains and you need do
nothing. Unfortunately, there are a lot of systems where no drains were
installed, and since they are buried, it would be hard to find them.

The trick with doing it yourself is that you need a high volume air
compressor (many home models are low volume), and a way to hook the air
compressor into the system.

My suggestion would be to hire a professional for this year, but insist
that you be able to watch him work. That way, you will see how to hook
up the compressor. If a lot of water comes out of the heads when he
injects the air, I would assume your system has no drains, and then you
will have to decide whether to install drains, or get your own
compressor, or continue using the professional. If, on the other hand,
nothing but air comes out of the heads, I would take that as evidence
that your system has drains, and you don't have to worry about blowing
it out each year.

Good suggestion.

I will point out however, that you don't need a high volume/low pressure
compressor. The pros do to make the job go as fast as possible, but it
is possible to blow out irrigation systems with much more modest equipment.

I blow mine out with a home built compressor made with a 3/4HP tankless
compressor coupled to a 4gal (now 10gal) air tank. The unit barely
manages 1CFM at 80PSI.

You need the tank to get the "burst" of high volume.

You'll also have to figure out how you're going to connect it.

I have a "Y"'d hose bib - one side is the 1" feed to the system,
the other side is an ordinary hose valve to a hose reel. I disconnect
the hosereel, and "backfeed" the compressor thru the valve, using
the valve to control flow. Just need a M air compressor fitting
to male hose connector to attach.

Takes about 40 minutes to do my 6 valve system (two main 1" branches,
perhaps 500' of line all told). The other system (2 valves plus a sink)
takes about 15 minutes.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.