Which product did you purchase out of interest.
if you have an open vented system, which I suspect you do as you mentioned a cw tank, you would normally drain down the system, put the desludging chemicals in the f&e tank and let the system refill.
You would then run the system for a period of time, as per manufacturers guidlines and then drain down again. You then refill and drain down until the water runs clear.
After the final drain down, put some inhibitor in the f&e tank and then refill, job done.
Make sure you put the chemicals in the correct tank, if you are unsure run the hot water for a while and see which tank fills. This is the hot water supply tank, your f&e tank will be the other one.
If you are using Fernox DS40 (superfloc) beware if your system is old, as it can be quite aggresive if left too long and you may end up with pin hole leaks in the heat exchanger or pipes. Sentinel X400 would be a safer bet is better as you can't overdose the system.