I flush my system about every two years.
I turn off all radiators except one, this is generally an upstairs or
highest radator.
Then I turn the C/H pump to the high speed. It has a high and a normal
speed.
Then connect a hose pipe to the drain cock and poke the other end down
a drain.
Then open the drain cock and observe the water coming out to see the
colour.
Make sure the header tank is working and refilling the system as it
drains.
I usually allow ten minutes of clear water to flow out before closing
that radiator and opening the next one.
Repaet the process for each radiator in turn.
I generally take a couple of weeks for the C/H water to completely
de-gas/air itself so you'll have to bleed the rads for a while.
Do this in the summer if possible.

Chris.