No, not petrol additives. These won't go through the inlet manifold and wash the valves, they will come through the fuel in minute quantities and only clean the injectors themselves as the fuel passes through the injectors. And I wouldn't bother with them at all, there ought to be something to do that in the fuel already.
Ok, you're looking for an aerosol can which says it cleans carbs and is to be squirted into the intakes. Halfords will sell some of these, they're all generally much the same thing, truth be told.
You take the air cleaner out.
You point it in while the engine is running and press the nozzle down so the stuff sprays out.
Pick a can that says this is what it is for, and it is to be used in this way. Carb / injection cleaner, makes no odds really as you're actually trying to wash debris off a valve.
If the engine is running I can't imagine what might go wrong, but check what the can says just in case.
If it is a worry, just go straight to the mechanic's solution instead, this is only a cheap and easy way to try a simpler solution - it's quite likely to achieve nothing at all.
P.S. I do this regularly on my car, mainly to clean the airflow sensor. It's a hot wire that has gunk burn onto it and gradually reads higher and higher. So far, nothing bad has happened. Since you are pumping corrosive petrol into the same places, I doubt it's going to harm the combustion chamber even if you use fly spray or Mountain Breeze air freshener, but probably best not to experiment. ;)