Highflow 400?

I have highflow 400 BF boiler running my central heating system. It is mounted at floor level, and due to recent problems I have been adised to
flush the system out and use a de-sludger agent.
I din't know how and where to drain and introduce the de-sludger.
I know the boiler resonably well having in the past changed the divertor valve, pump and lower manifold. Can anyone give me some expert advice?