Assuming you do need to add it:
1. It doesn't have to be via a radiator.
2. Add it with the system turned off.
3. If you have a filter like a Magnaclean near the boiler, you can add it via that. Turn off the valves on the inlet to and outlet from the filter, remove the lid, siphon / sponge out the water and pour in the Sentinel. Turn the valves on and run the system for 10 minutes. Probably repeat because you can't get a whole litre in the first time. Not all filters have removable lids.
4. If you have a towel rail / radiator on the system you can add it via that. Close both valves at the base of the radiator. Undo the bleed nipple, and bleed until no more water comes out. Undo the big nut with the bleed nipple in the middle of it, and remove. Syphon as much water as you can out of the radiator, and add the Sentinel. Replace the big nut and bleed nipple, make sure bleed nipple is closed, open valves and run the system.
5. If you have to do it through a radiator, the principle and practice is the same as for a towel rail, but its generally a bit more awkward. Also, there may be no "big nut" on the top of the radiator, particularly if old.
6. If you are using X400 you can get the chemical in a "rapid dose" container. You still have to release system pressure (e.g. via a bleed nipple) but then just follow the instructions. I have never personally found these much use, but its probably me.
7. However you do it, it is worth bleeding all the radiators when the system has gone cold, and bringing the boiler pressure back up to its usual value (around 1.3 bar) as necessary.
8. When you have finished flushing the system, you will need to drain it entirely. How long it stays in depends on what cleaner you are using. Once flushed and drained, refill with clean water and add an inhibitor.