The chemical might be boiler noise reducer, corrosion inhibitor, or cleanser – the procedure is the same.
One person can do this but it’s handier with two.
Locate a drain valve for the heating system (often on a radiator close to a door, sometimes on an outer wall) and attach a hose (perhaps also a jubilee clip to secure it and prevent leaks) and run this to outside. It can be useful to have an old towel under the drain valve. Have handy a plastic milk carton or similar clear or translucent container to enable a small sample of the drained water to be taken.
Obtain some string or a bungee cord and a torch, and of course the chemical.
Carrying the chemical and torch, gain access to the loft and the small feed & expansion (“F&E” or “header”) tank (not the much larger cold water storage cistern).
Pour in the chemical and either tie up the float (ball) valve or turn off the tap if there is one, to stop the tank filling.
Open the drain valve and check that water is leaving the system. Collect a small sample.
In the loft, monitor the level of water in the F&E tank as it falls. When the surface of the water is just above the outlet pipe near the bottom of the tank, open the tap or release the float valve to allow the tank to refill. (Letting the level fall too low will draw air into the system, and oxygen in air causes corrosion in heating system water.)
As soon as possible (this is why it’s handy to have two people), close the drain valve and detach the hose.
Close the loft and tidy up – job done.
It can be useful to make a note of the date, perhaps by marking the empty chemical container.
Take a look at the water sample, which should be clear – if it’s black, corrosion has been taking place.