I'm not very scientific about flushing coolant.  I just unhook the two main
lines on the left of the engine and flush in one and out the other.  Then I
unhook the two heater lines on the right of the engine and flush through the
block using the supply line that feeds the waterpump.   Now turn on the
heater controls and flush in one heater line and out the other.  Then go up
front and unhook the two lines going to the radiator.  Flush into one and out
the other.  Also flush down both of the supply steel lines from the engine
compartment.  Now everything is flushed and there is coolant/water all over
the place.  When you put everything back together after all the water has
drained out, there will be aproximately two gallons of water in the system
(it hides in lines and in the block, all over the place).  So I hook all of
the lines back up and  just add two gallons of pure anti-freeze and let the
stuff mix itself inside the motor.  If you need to add a little more than two
gallons then top if off with water (but usually it turns out just about
right).  Can you see why I don't like to flush coolant in the winter?  This
description is for 1.9l Vanagons.  I'm going to flush my 2.1l here pretty
soon but the concept is the same.  What you really want to do is to flush out
the block as best you can.  Draining it from those little drain holes on the
bottom of the motor just doesn't cut it.  You want to squirt water in there
with some pressure to get all that crap out of there (and believe me there is
some junk in there, most of which won't come out anyway).
Have fun and turn this into a fun project where you are not afraid to get wet
(pick a warm day and wear shorts, and an old T-shirt.  Please drain old
coolant before you start and properly dispose of it (I usually strain it and
use it to top off coolant on vehicles that are old and don't care what
coolant is in them anyway or leave it out for cats I don't like, to enjoy:).
Please Bob I am just kidding about the cats!
Have fun and don't get any on ya!
Ken Wilford
Van-Again
John 3:16