Easy way, switch off water supply to header or tie up ball valve. drain out water from system until header is empty using the drain point. add inhibitor, switch on water supply again to header. run system to circulate. This is not the best way as not all your inhibitor will be in the system.
Alternate way is to get your system protector in a tube like silicone, ferinox do this, they also provide the instructions, switch off a rad, remove bleed point, attatch tube, open one end of rad and inject in, switch off rad, remove tube, refit bleed point. run system for a while.
This will get inhibitor right into the heart of the system.