First you should drain the system down and add a central heating cleanser, fill the system back up and let it run for a couple of hrs. Then drain the system down again fill it up with fresh water run for another hr and drain it down again. Then put in the inhibitor, fill the sysem up again and the job is done. You can put the cleanser and the inhibitor into the system by pooring it into the header tank in the loft after you have drained it down so it goes into the system and not just into the header tank. If you do not have a header tank in the loft or airing cupboard then you may have a sealed system or a combi etc... In this case you can add the cleanser and the inhibitor to any radiator via the air bleeding nipple on the radiators using a special dosing gun or even remove the air bleed valve fully and just poor it in using a funnel.