Here are my install and flush procedures.  It has worked well for me for three years now, but of course, I make no guarantee that this will work for you.

Installation:

1.  Tee into the hose between the engine saltwater intake and the engine.  In my case, I wasn't using the saltwater pump in the galley sink much, and not being sure if this whole idea would even work,  I capped the saltwater galley pump and teed into that hose at the engine intake instead.  If I had it to do over, I would just tee into the engine saltwater intake line instead.  The bottom line is that however you do it, you just want the freshwater from your bucket to be sucked into the engine's freshwater pump while the engine thru-hull is shut off (so no saltwater can get in the system).

2.  Out of the tee connect a hose, and to that hose connect a simple plumbing shutoff valve. I got mine at a local hardware store.  (West Marine quality/prices might be overkill on this one.)

3.  To the exit side of the hose which has the plumbing shutoff valve, teflon-tape and screw in a simple garden hose male fitting. I also bought this at a local hardware store.  See first attached picture.
 
4.  Buy a cheap, short (like say 6 foot) garden hose and a cheap 5-gallon bucket at a hardware store.

5.  Buy Saltaway on Amazon.com or whatever. I got the large economy size.  Definitely NOT cheap, but it lasts a year or so if this is all you are using it for, so no biggie there.

That is your whole system, assuming you already have a hose from the dock water to your boat (which most people do for boat washdowns and filling the water tanks and such).

Procedure for Flushing the Engine:

1.  Screw the female connector of the 6-foot garden hose to the male connector on the freshwater flush hose (leading to the engine intake hose).  (I love that part for some odd reason. :-)) 

2.  REALLY IMPORTANT: Open the freshwater flush valve.  (Be sure to keep the engine thru-hull seacock closed, since the whole point is to flush the engine with only freshwater.

3.  Fill the bucket about half full with dock freshwater. 

4.  Insert the 6-foot garden hose into the bucket so that the end stays at the bottom of the bucket.  It is obviously important to keep this hose at the bottom of the bucket pulling fresh water instead of air, so keep an eye on it while the engine is flushing! See second picture.

5.  Have your dock freshwater hose ready to start feeding water into the bucket, but don't start it yet.

6.  Start your engine.

7.  Immediately put your finger at the end of the 6-foot garden hose in the bucket to make sure the engine water pump is sucking water in well.  If it's not, shut off the engine immediately and figure out what's wrong.

8.  Immediately after that, start the dock freshwater hose feeding water into the bucket.  You want the flow rate into the bucket to be equal to the flow rate being pulled into the engine intake hose.  This will keep the bucket about half full, giving you a "buffer" either way, in case you have to step away for a minute or two (to find more male/female garden hose connectors or whatever :-)).  That is to say, you don't want the bucket to overflow and neither do you want it to empty completely and have the engine water pump suck air.  As Egon said about crossing the beams in the movie Ghostbusters, "That would be bad, right?".

9.  Let the engine run in this state for about 10 minutes, to bring the entine up to normal operating temperature and flush all the salt out of the engine.

10.  As added insurance for complete salt removal from the engine, pour an ounce or two of Saltaway into the bucket.  It's concentrated, so it doesn't need much. Read the directions for exact amount to use.

11.  Shut off the dock water hose.  This will empty the bucket with the Saltaway into the engine water system.

12.  Watch until the water in the bucket gets down to about 1/4 of the bucket.  Immediately go to the rear of the boat. You should see soapy suds pouring out of the exhaust outlet.  This indicates that the Saltaway has thoroughly saturated the engine water-cooling system.  This is what you want.

13.  IMPORTANT:  After you see suds out of the exhaust hose thru-hull fitting, but BEFORE the bucket is empty, i.e. before the engine water pump starts sucking air at the bucket, shut off the engine.  Note that you have a bit of time to get back to the engine and shut it off if you are watching the bucket carefully.  You want as much Saltaway to pass through the engine freshwater cooling system as possible (to eat away any remaining salt), but without sucking air, so this is a bit of a "dance". 

14.  Immediately, shut off the freshwater flush valve.  What you do NOT want is for water from the bucket to siphon into the engine.  If that goes on long enough, it could siphon all the way into the cylinders (though thinking about it all, the odds of that happening look pretty slim at this point...but why take unnecessary chances?).

15.  Unscrew the 6-foot garden hose and put everything away for next time. BE SURE TO CLOSE THE FRESHWATER VALVE!! or when you run your engine at sea, your engine impeller will be sucking air!

Some important points:

1.  Never attach a pressurized dockwater hose to the freshwater flush system, thinking that you can eliminate the bucket.  Someone on this forum pointed out that if the pressure is greater than the intake capacity of the freshwater engine pump, water could eventually enter the cylinders!  Why take that risk? Use the bucket.

2.  For a similar reason, never leave the bucket with a significant amount of water in it, with the freshwater flush valve open.  Again, water could eventually siphon into the cylinders!

3.  The reason you want to END with the Saltaway, is that over time (days, weeks), the Saltaway will sit in there and eat up any remaining salt crystals in the engine.