This is one of those jobs where you have to get creative. One requirement is access to compressed air and solvent. This job is best done with all the a/c components broken down to individual pieces. I usually pour solvent into the unit and place the air gun nozzle tip against the hole to flush the solvent. I use a funnel with hose on the bottom for pouring. I dress the tip of the air gun with a short piece of rubber hose to provide a seal while shooting air through the unit.
A lot of people use mineral spirits for the bulk of the flush, then follow up with lacquer thinner, brake cleaner, carb cleaner, or alcohol to remove the residue left by mineral spirits.
All "open" pieces get flushed. The drier gets replaced. The compressor is taken care of by first draining it, then pouring in some of the new lubricant and working it through by cranking the compressor by hand. I'm not sure how to handle the expansion orifice - it depends on the particular design (some are insertable) and whether there's crud in the system, or if you're just removing old lubricant.
All gaskets and o-rings should be replaced. I like to dress connections with scotchbrite for a good sealing surface. The parent lubricant, or better yet, Nylog, available from Carlisle Auto Air
or AC Source
should be used to lubricate the connections.